You can install pre-built binaries from https://repo.dovecot.org/

Docker images can be found at https://hub.docker.com/r/dovecot/dovecot

Please review https://doc.dovecot.org/2.4.2/installation/upgrade/2.3-to-2.4.html and https://doc.dovecot.org/2.4.2/installation/installation.html.

Important

The v2.4.2 comes with new libpcre2 dependency.

Critical bug fixes

• CVE-2025-30189: Passdb oauth2 (not oauth2 mechanism), passdb passwd,
  passdb bsdauth, and userdb passwd drivers would cause users to be
  cached with same cache key when auth cache was enabled.

Changes

• auth: Remove proxy_always field.
• config: Change settings history parsing to use python3.
• doveadm: Print table formatter - Print empty values as "-".
• imapc: Propagate remote error codes properly.
• lda: Default mail_home=$HOME environment if not using userdb lookup
• lib-dcrypt: Salt for new version 2 keys has been increased to 16 bytes.
• lib-dregex: Add libpcre2 based regular expression support to Dovecot,
  if the library is missing, disable all regular expressions. This
  adds libpcre2-32 as build dependency.
• lib-oauth2: jwt - Allow nbf and iat to point 1 second into future.
• lib: Replace libicu with our own unicode library. Removes libicu as build
  dependency.
• login-common: If proxying fails due to remote having invalid SSL cert, don't reconnect.

New features

• auth: Add ssl_client_cert_fp and ssl_client_cert_pubkey_fp fields, see
  https://doc.dovecot.org/latest/core/summaries/settings.html#ssl_peer_certificate_fingerprint_hash
  for more information.
• config: Add support for $SET:filter/path/setting.
• config: Improve @group includes to work with overwriting their settings.
• doveadm kick: Add support for kicking multiple usernames
• doveadm mailbox status: Add support for deleted status item.
• imap, imap-client: Add experimental partial IMAP4rev2 support.
• imap: Implement support for UTF8=ACCEPT for APPEND
• lib-oauth2, oauth2: Add oauth2_token_expire_grace setting.
• lmtp: lmtp-client - Support command pipelining.
• login-common: Support local/remote blocks better.
• master: accept() unix/inet connections before creating child process
  to handle it. This reduces timeouts when child processes are slow to
  spawn themselves.

Bug fixes

• SMTPUTF8 was accepted even when it wasn't enabled.
• auth, *-login: Direct logging with -L parameter was not working.
• auth: Crash occured when OAUTH token validation failed with
  oauth2_use_worker_with_mech=yes.
• auth: Invalid field handling crashes were fixed.
• auth: ldap - Potential crash could happen at deinit.
• auth: mech-gssapi - Server sending empty initial response would cause
  errors.
• auth: mech-winbind - GSS-SPNEGO mechanism was erroneously marked as
  not accepting NUL.
• config: Multiple issues with $SET handling has been fixed.
• configure: Building without LDAP didn't work.
• doveadm: If source user didn't exist, a crash would occur.
• imap, pop3, submission, imap-urlauth: USER environment usage was broken
  when running standalone.
• imap-hibernate: Statistics would get truncated on unhibernation.
• imap: "SEARCH MIMEPART FILENAME ENDS" command could have accessed
  memory outside allocated buffer, resulting in a crash.
• imapc: Fetching partial headers would cause other cached headers to
  be cached empty, breaking e.g. imap envelope responses when caching to disk.
• imapc: Shared namespace's INBOX mailbox was not always uppercased.
• imapc: imapc_features=guid-forced GUID generation was not working correctly.
• lda: USER environment was not accepted if -d hasn't been specified.
• lib-http: http-url - Significant path percent encoding through parse
  and create was not preserved. This is mainly important for Dovecot's
  Lua bindings for lib-http.
• lib-settings: Crash would occur when using %variables in SET_FILE type settings.
• lib-storage: Attachment flags were attempted to be added for
  readonly mailboxes with mail_attachment_flags=add-flags.
• lib-storage: Root directory for unusable shared namespaces was
  unnecessarily attempted to be created.
• lib: Crash would occur when config was reloaded and logging to syslog.
• login-common: Crash might have occured when login proxy was destroyed.
• sqlite: The sqlite_journal_mode=wal setting didn't actually do anything.
• Many other bugs have been fixed.

Installation

You can install pre-built binaries from https://repo.dovecot.org

Docker images can be found at https://hub.docker.com/r/dovecot/dovecot

Please review https://doc.dovecot.org/2.4.1/installation/upgrade/2.3-to-2.4.html and https://doc.dovecot.org/2.4.1/installation/installation.html.

Important

We have changed the signing key for 2.4 going forward, releases are signed with EF0882079FD4ED32BF8B23B2A1B09EF84EDC5219, which can be found at https://repo.dovecot.org/DOVECOT-REPO-GPG-2.4 and is signed with the previous key.

The old key has been renamed to https://repo.dovecot.org/DOVECOT-REPO-GPG-2.3.

We are happy to provide experimental arm64 support in the form of a Docker image. There are now three kinds of images, latest, latest-dev and latest-root.

The latest docker image is now ran rootless, with UID 1000 as vmail. Please take this into consideration when upgrading. The latest-root image still runs as root. Latest 2.3 image can be used with tag 2.3-latest still.

Changes

• auth: Change unix_listener/auth-userdb/group = $SET:default_internal_group
  This change needs dovecot_config_version=2.4.1.
• auth: lua - Remove support for single string result.
• imap: Unconditionally advertise SPECIAL-USE capability.
• lib-dcrypt: Install dcrypt_openssl.so into dovecot modules directory.
• lib-master: For glibc, default MALLOC_MMAP_THRESHOLD_=131072.
• lib-storage: Change default mail_cache_fields to:
  hdr.date hdr.subject hdr.from hdr.sender hdr.reply-to hdr.to
  hdr.cc hdr.bcc hdr.in-reply-to hdr.message-id
  date.received size.virtual imap.bodystructure mime.parts hdr.references
  hdr.importance hdr.x-priority hdr.x-open-xchange-share-url
  pop3.uidl pop3.order. This change needs dovecot_config_version=2.4.1.
• lib-var-expand: Use moduledir instead of pkglibdir for crypt.
• lmtp: Change the default lmtp_user_concurrency_limit to 10.
  This change needs dovecot_config_version=2.4.1.
• lmtp: Change the default service_restart_request_count to 1.
  This change needs dovecot_config_version=2.4.1.

New features and additions

• auth: Allow configuring passdb/userdb sql to use auth-workers.
• config: Add default group @mailbox_defaults = english.
• config: Improve "Unknown setting" error with more details and
  suggestions.
• doveconf: Add -U parameter to ignore unknown settings in config file.
• fts-flatcurve: Support lock files in VOLATILEDIR.
• imap-acl: Add support for the IMAP LIST-MYRIGHTS capability (RFC 8440).
• imap-client: Support ANONYMOUS authentication.
• imap: Implement support for the REPLACE capability.

Bug fixes

• auth: ldap - Passdb fields were ignored with
  passdb_ldap_bind_userdn=yes.
• auth: lua - Fix error result handling in lua passdb/userdb.
• auth: oauth2 - When building oauth2 failure reply, memory would leak.
• config: local_name handling would work wrong with multiple names and
  wildcards.
• fts-flatcurve: A potential crash could occur when searching virtual
  mailboxes.
  Fixes: Panic: file fts-search.c: line 87 (level_scores_add_vuids):
  assertion failed: (array_count(&vuids_arr) == array_count(&br->scores))
• fts-flatcurve: Maybe queries were done wrong.
• fts-flatcurve: Non-selectable mailboxes were not ignored when doing
  optimize/rescan.
• fts-flatcurve: Signal 11 crash could happen with fts rescan.
• fts: Fix crash caused by event object lifecycle mishandling.
• imap-hibernate: Client counters would get reset on unhibernation,
  affecting imap_logout_format variables.
• imap: Crash would occur with Maildir when trying to send INPROGRESS
  during mailbox syncing.
• ldap: Dovecot could not be compiled without LDAP.
• lib-dcrypt: Output stream encryption can cause assert crash if
  attempting to encrypt over 64 GiB of data with GCM. This is still not
  supported with GCM, but it fails better.
• lib-http: HTTP client context memory usage was increasing.
• lib-http: Pipeline corruption could happen after 100 Continue response.
• lib-settings: Variable expansion initialization could crash with
  Panic: file settings.c: line 1560 (settings_var_expand_init_add):
  assertion failed: (I_MAX(num_tables, num_provs) == num_ctx)
• lib-smtp: Pipelining initial SASL response after AUTH was broken.
• lib-var-expand: If filter failed, memory leak would occur.
• lib-var-expand: Older bison versions did not have error symbol for
  handling causing unexpected behaviour on the parser on error conditions.
• quota: Quota calculations had minor bugs causing small errors.

Installation

You can install pre-built binaries from https://repo.dovecot.org

Docker images can be found at https://hub.docker.com/r/dovecot/dovecot

Please review https://doc.dovecot.org/2.4.0/installation/upgrade/2.3-to-2.4.html and https://doc.dovecot.org/2.4.0/installation/installation.html.

Important

We have changed the signing key for 2.4 going forward, releases are signed with EF0882079FD4ED32BF8B23B2A1B09EF84EDC5219, which can be found at https://repo.dovecot.org/DOVECOT-REPO-GPG-2.4 and is signed with the previous key.

The old key has been renamed to https://repo.dovecot.org/DOVECOT-REPO-GPG-2.3.

We are happy to provide experimental arm64 support in the form of a Docker image.

Docker images are now run rootless, with UID 1000 as vmail. Please take this into consideration when upgrading. Latest 2.3 image can be used with tag 2.3-latest still.

Changes

• config: dovecot_config_version must be the first non-comment
  line in configuration file.
• config: dovecot_storage_version must be in the configuration
  file.
• config: Many configuration options have changed so old configuration
  files do not work without rewrite. See
  https://doc.dovecot.org/latest/installation/upgrade/2.3-to-2.4.html
• config: New variable expansion syntax has been introduced, see
  https://doc.dovecot.org/latest/core/settings/variables.html
• config: Some default settings have changed.
• config: plugin {} section has been removed.
• *-login: With ssl=required, connections from login_trusted_networks
  are now also required to be SSL/TLS encrypted.
• acl: Use ACL settings instead of Global ACL Directories.
• auth-worker: auth_worker_max_count is replaced with
  service auth-worker { process_limit }.
• auth: Weak password schemes are disabled by default, use
  auth_allow_weak_schemes to enable them.
• auth_debug, mail_debug: Use log_debug filter instead.
• config: All sections require a name, for example passdb/userdb:

  passdb static {
    password=secret
  }
  

• db2: Remove Berkeley DB support.
• dict-memcached: This is removed, use Redis instead.
• director: Feature has been removed. Unsupported small-scale replacement:
  https://github.com/dovecot/tools/blob/main/director.lua
• doveadm: USER environment variable is only supported with
  --no-userdb-lookup. One of -u, -F or -A must be used
  otherwise.
• doveconf: Option -n is now default when running doveconf.
• dsync: Use doveadm sync instead, legacy symlink has been removed.
• fs-sis: Feature is now deprecated and has been made read-only.
  It will be removed in future release.
• fts-lucene, fts-squat: These have been removed, use fts-flatcurve or
  fts-solr instead.
• imap-login: IMAP compression is now handled in proxies.
• imap_quota: SETQUOTA / quota_set has been removed.
• imap_zlib: This plugin is no longer needed, it's always enabled.
• imapc: All features are enabled by default, imapc_features can be used
  to explicitly disable features that are not wanted.
• lib-storage: mbox driver is now frozen.
• mail_compress: XZ and LZMA algorithm support has been removed.
• mailbox-alias: Plugin has been removed.
• old_stats, auth_stats: These have been removed.
• openssl: Minimum supported version of OpenSSL is now 1.1.1.
• openssl: Add support for OpenSSL 3.x
• quota-dict, quota-dirsize: These have been removed, use quota-count
  instead. You can use quota-clone to copy quota usage to some database.
• replicator: Feature has been removed. Use NFS or some other shared
  filesystem instead, or run doveadm sync in crontab.
• stats: The bytes_in and bytes_out field in several events have been
  renamed as net_in_bytes and net_out_bytes.
• zlib: Renamed to mail_compress plugin.

New features and additions

• Experimental SMTPUTF8 and IMAP UTF8=ACCEPT support has been added.
  Needs --enable-experimental-mail-utf8 configure option and
  mail_utf8_extensions=yes setting.
• Long running mail commands can be aborted with Ctrl-C / doveadm kick.
• auth: LDAP driver now supports multi-value attributes.
• auth: Add support for SCRAM-SHA-1-PLUS and SCRAM-SHA-256-PLUS.
• auth: Add support for TLS channel binding.
• auth: Support sending JA3 hash to policy server.
• configure: Detect latest Lua version.
• *-login: Support for TLS Server Name has been improved to allow pre-login
  settings. For example capabilities to be changed based on TLS Server Name.
• *-login: Support for TLS ALPN has been added, connections with mismatching
  application are now refused. Missing ALPN is accepted.
• fts-flatcurve: New Xapian based FTS plugin has been added.
• imap: Support for INPROGRESS untagged messages as per RFC 9585.
• lib-lua: Expose Dovecot DNS client.
• lib-lua: Expose Dovecot HTTP client.
• lib-sasl: Support SCRAM-SHA mechanisms.
• lmtp: SNI support has been added which allows settings to be applied
  based on TLS Server Name.
• sqlite: Support WAL mode.
• stats: Submetric name size has been increased.
• submission: Add submission_add_received_header setting to protect
  sender identity by suppressing the Received: header.

Bug fixes

• Many bugs have been fixed.

• CVE-2024-23184: A large number of address headers in email resulted
  in excessive CPU usage.
• CVE-2024-23185: Abnormally large email headers are now truncated or
  discarded, with a limit of 10MB on a single header and 50MB for all
  the headers of all the parts of an email.
• oauth2: Dovecot would send client_id and client_secret as POST parameters
  to introspection server. These need to be optionally in Basic auth
  instead as required by OIDC specification.
• oauth2: JWT key type check was too strict.
• oauth2: JWT token audience was not validated against client_id as
  required by OIDC specification.
• oauth2: XOAUTH2 and OAUTHBEARER mechanisms were not giving out
  protocol specific error message on all errors. This broke OIDC discovery.
• oauth2: JWT aud validation was not performed if aud was missing
  from token, but was configured on Dovecot.

• lib-oauth2: Allow JWT tokens to be validated with missing typ field.
  The typ field is left out by some key issuers to conserve space,
  notably kubernetes. Now missing typ is tolerated, but if present, it
  still must be "jwt".

• auth: Auth passdb and userdb reply can contain "event_=value"
  which will be added to login event and mail user event respectively.
• lib-master: Set process title during various initialization stages to
  clearly describe what the process is waiting on.
• lib-storage: The mail_temp_scan_interval is now fuzzed incrementing it
  by 0..30% based on username's hash to reduce the chance of load spikes
  .
• lib-storage: The temp file scan has been moved from the open of the
  mailbox to the close, to reduce the latency perceived by users.
• stats: If metric has fields specified, all these fields are
  exported as counters to prometheus exposition.
  See https://doc.dovecot.org/configuration_manual/stats/openmetrics/.

• *-login: Processes might have crashed when a SSL connection disconnect
  s
  uncleanly.
• acl: When plugin was loaded \HasChildren and \HasNoChildren flags
  were calculated incorrectly for mailboxes containing '*' and '%'
  in their names.
• auth: Crash occured if a connection to PostgreSQL database server
  failed during startup.
• auth: Logins with invalid passwords (e.g. unknown scheme) in passdb
  were failing with "password mismatch" instead of "internal error".
• auth: XOAUTH2 and OAUTHBEARER mechanisms were not giving out protocol
  specific error message on all errors. This especially broke OIDC
  discovery.
• dbox: When last_temp_file_scan header wasn't set (especially after
  dsync migration), the next mailbox open always triggers the temp file
  scan. This could have caused a load spike after migrations. Fixed by
  using the mailbox directory's atime when the header isn't set, which
  usually moves the scan time into the future.
• dict-redis: A crash would occur on transaction rollback.
• dsync: Infinite loop causing out of memory would occur when handling
  mailbox deletion from remote end and hierarchy separators would differ.
• dsync: Incremental dsync failed for folder names ending with '%',
  unless BROKENCHAR was set. Also folder names with '%' elsewhere in
  them caused each incremental dsync to unnecessarily rename the folder
  to a temporary name and back. v2.3.19 regression.
• imap-hibernate: If an IMAP client unhibernation timed out with
  "(version received)", the unhibernation could still have successfully
  finished later on and continued working normally. This was rather
  confusing, because imap-hibernate already logged that the client got
  disconnected. Avoid this by forcing the connection to shutdown on
  unhibernation timeout.
• imapc: Crashed when a folder mapped through the virtual plugin
  disappears from the storage.
• imapc: EXPUNGE, EXISTS or FETCH replies from a server for a previously
  selected mailbox could have been processed as if they belonged to the
  new mailbox currently being selected. This could have caused warnings.
• lib-http: Dovecot HTTP server (doveadm, stats/openmetrics) may have
  disconnected HTTP clients before the response is fully sent. This
  happened only on busy servers where kernel's socket buffers were
  rather full.
• lib-http: Fixed a potential crash on http-server if a client
  disconnected early. v2.3.18 regression.
• lib-index: Index file corruption could have caused a crash. Fixes:
  Panic: file mail-transaction-log-view.c: line 165 (mail_transaction_log_view_set):
  assertion failed: (min_file_seq <= max_file_seq).
• lib-index: Purging an existing >1GB cache file can crash. Now cache
  files still above 1GB after purging are removed. Fixes:
  Panic: file mail-index-util.c: line 10 (mail_index_uint32_to_offset):
  assertion failed: (offset < 0x40000000)
• lib-lua: A HTTP client could not resolve DNS names in mail processes,
  because it expected "the dns-client" socket to exist in the current
  directory.
• lib-oauth2: Dovecot would send client_id and client_secret as POST
  parameters to the introspection server. These need to be optionally in
  Basic auth instead.
• lib-oauth2: JWT aud validation was not performed if aud was missing
  from a token, but was configured on Dovecot.
• lib-oauth2: JWT key type check was too strict.
• lib-oauth2: JWT token audience was not validated against client_id as
  required by the specification.
• lib-ssl-iostream: Using the ssl_require_crl=yes setting may have caused
  CRL check failures for outgoing SSL/TLS connections, although it was
  supposed to affect checking CRLs only for client-side SSL
  certificates. v2.3.17 regression.
• lib-sql: MySQL driver leaked memory when connection failed.
• lib-storage: Various fixes when running into out of disk space.
• master: Service idle_kill setting didn't work properly on busy
  servers. It was very unlikely that any process was idling long enough
  to become killed. Also the idle_kill handling code was using quite a
  lot of CPU on the master process when there were a lot of processes
  (e.g. imap). The new behavior is to track the lowest number of idling
  processes every idle_kill time interval and then kill that many idling
  processes.
• mdbox: Temp file scan was done for always empty directories.
• mdbox: The fdatasync() call was done in wrong parent directory when
  writing mails. Also on a failure it crashed instead of logging an error.
• notify_status: The plugin crashes if any user initialization fails.
• pop3: Sending command with the ':' character caused an assert-crash.
  v2.3.18 regression. Fixes: Panic: event_reason_code_prefix(): name has ':'
• stats: Fix panic when a nonexistent event exporter was referenced while
  adding a new metric dynamically via doveadm stats add. This produces
  a proper error now.
• stats: If process exported a lot of events and then exited, some of
  the last events may have become lost.
• stats: Invalid Prometheus label names were created with specific
  histogram group_by configurations. Prometheus rejected these labels.
• welcome: The plugin didn't execute in some situations that created
  INBOX but didn't open it, e.g. if GETMETADATA was used before the
  INBOX was opened.

• Add dsync_features=no-header-hashes. When this setting is enabled and
  one dsync side doesn't support mail GUIDs (i.e. imapc), there is no
  fallback to using header hashes. Instead, dsync assumes that all mails
  with identical IMAP UIDs contains the same mail contents. This can
  significantly improve dsync performance with some IMAP servers that
  don't support caching Date/Message-ID headers.
• lua: HTTP client has more settings now, see
  https://doc.dovecot.org/admin_manual/lua/#dovecot.http.client
• replicator: "doveadm replicator status" command now outputs when the
  next sync is expected for the user.

• LAYOUT=index: duplicate GUIDs were not cleaned out. Also the list
  recovery was not optimal.
• auth: Assert crash would occur when iterating multiple userdb
  backends.
• director: Logging into director using master user with
  auth_master_user_separator character redirected user to a wrong
  backend, unless master_user_separator setting was also set to the same
  value. Merged these into auth_master_user_separator.
• dsync: Couldn't always fix folder GUID conflicts automatically with
  Maildir format. This resulted in replication repeatedly failing
  with "Remote lost mailbox GUID".
• dsync: Failed to migrate INBOX when using namespace prefix=INBOX/,
  resulting in "Remote lost mailbox GUID" errors.
• dsync: INBOX was created too early with namespace prefix=INBOX/,
  resulting a GUID conflict. This may have been resolved automatically,
  but not always.
• dsync: v2.3.18 regression: Wrong imapc password with dsync caused
  Panic: file lib-event.c: line 506 (event_pop_global):
  assertion failed: (event == current_global_event)
• imapc: Requesting STATUS for a mailbox with imapc and INDEXPVT
  configured did not return correct (private) unseen counts.
• lib-dict: Process would crash when committing data to redis without
  dict proxy.
• lib-mail: Corrupted cached BODYSTRUCTURE caused panic during FETCH.
  Fixes: Panic: file message-part-data.c: line 579 (message_part_is_attachment):
  assertion failed: (data != NULL). v2.3.13 regression.
• lib-storage: mail_attribute_dict with dict-sql failed when it tried to
  lookup empty dict keys.
• lib: ioloop-kqueue was missing include breaking some BSD builds.
• lua-http: Dovecot Lua HTTP client could not resolve DNS names in mail
  processes, because it expected "dns-client" socket to exist in the
  current directory.
• oauth2: Using %{oauth2:name} variables could cause useless
  introspections.
• pop3: Sending POP3 command with ':' character caused an assert-crash.
  v2.3.18 regression.
• replicator: Replication queue had various issues, potentially causing
  replication requests to become stuck.
• stats: Invalid Prometheus label names were created with specific
  histogram group_by configurations. Prometheus rejected these labels.

• doveadm deduplicate: Non-duplicate mails were deleted.
  v2.3.19 regression.
• auth: Crash would occur when iterating multiple backends.
  Fixes: Panic: file userdb-blocking.c:
  line 125 (userdb_blocking_iter_next): assertion failed: (ctx->conn != NULL)

• Added mail_user_session_finished event, which is emitted when the mail
  user session is finished (e.g. imap, pop3, lmtp). It also includes
  fields with some process statistics information.
  See https://doc.dovecot.org/admin_manual/list_of_events/ for more
  information.
• Added process_shutdown_filter setting. When an event matches the filter,
  the process will be shutdown after the current connection(s) have
  finished. This is intended to reduce memory usage of long-running imap
  processes that keep a lot of memory allocated instead of freeing it to
  the OS.
• auth: Add cache hit indicator to auth passdb/userdb finished events.
  See https://doc.dovecot.org/admin_manual/list_of_events/ for more
  information.
• doveadm deduplicate: Performance is improved significantly.
• imapc: COPY commands were sent one mail at a time to the remote IMAP
  server. Now the copying is buffered, so multiple mails can be copied
  with a single COPY command.
• lib-lua: Add a Lua interface to Dovecot's HTTP client library. See
  https://doc.dovecot.org/admin_manual/lua/ for more information.

• auth: Cache lookup would use incorrect cache key after username change.
• auth: Improve handling unexpected LDAP connection errors/hangs.
  Try to fix up these cases by reconnecting to the LDAP server and
  aborting LDAP requests earlier.
• auth: Process crashed if userdb iteration was attempted while auth-workers
  were already full handling auth requests.
• auth: db-oauth2: Using %{oauth2:name} variables caused unnecessary
  introspection requests.
• dict: Timeouts may have been leaked at deinit.
• director: Ring may have become unstable if a backend's tag was changed.
  It could also have caused director process to crash.
• doveadm kick: Numeric parameter was treated as IP address.
• doveadm: Proxying can panic when flushing print output. Fixes
  Panic: file ioloop.c: line 865 (io_loop_destroy): assertion failed:
  (ioloop == current_ioloop).
• doveadm sync: BROKENCHAR was wrongly changed to '_' character when
  migrating mailboxes. This was set by default to %, so any mailbox
  names containing % characters were modified to "_25".
• imapc: Copying or moving mails with doveadm to an imapc mailbox could
  have produced "Error: Syncing mailbox '[...]' failed" Errors. The
  operation itself succeeded but attempting to sync the destination
  mailbox failed.
• imapc: Prevent index log synchronization errors when two or more imapc
  sessions are adding messages to the same mailbox index files, i.e.
  INDEX=MEMORY is not used.
• indexer: Process was slowly leaking memory for each indexing request.
• lib-fts: fts header filters caused binary content to be sent to the
  indexer with non-default configuration.
• doveadm-server: Process could hang in some situations when printing
  output to TCP client, e.g. when printing doveadm sync state.
• lib-index: dovecot.index.log files were often read and parsed entirely,
  rather than only the parts that were actually necessary. This mainly
  increased CPU usage.
• lmtp-proxy: Session ID forwarding would cause same session IDs being
  used when delivering same mail to multiple backends.
• log: Log prefix update may have been lost if log process was busy.
  This could have caused log prefixes to be empty or in some cases
  reused between sessions, i.e. log lines could have been logged for the
  wrong user/session.
• mail_crypt: Plugin crashes if it's loaded only for some users. Fixes
  Panic: Module context mail_crypt_user_module missing.
• mail_crypt: When LMTP was delivering mails to both recipients with mail
  encryption enabled and not enabled, the non-encrypted recipients may
  have gotten mails encrypted anyway. This happened when the first
  recipient was encrypted (mail_crypt_save_version=2) and the 2nd
  recipient was not encrypted (mail_crypt_save_version=0).
• pop3: Session would crash if empty line was sent.
• stats: HTTP server leaked memory.
• submission-login: Long credentials, such as OAUTH2 tokens, were refused
  during SASL interactive due to submission server applying line length
  limits.
• submission-login: When proxying to remote host, authentication was not
  using interactive SASL when logging in using long credentials such as
  OAUTH2 tokens. This caused authentication to fail due to line length
  constraints in SMTP protocol.
• submission: Terminating the client connection with QUIT command after
  mail transaction is started with MAIL command and before it is
  finished with DATA/BDAT can cause a segfault crash.
• virtual: doveadm search queries with mailbox-guid as the only parameter
  crashes: Panic: file virtual-search.c: line 77 (virtual_search_get_records):
  assertion failed: (result != 0)

• Removed mail_cache_lookup_finished event. This event wasn't especially
  useful, but it increased CPU usage significantly.
• fts: Don't index inline base64 encoded content in FTS indexes using
  the generic tokenizer. This reduces the FTS index sizes by removing
  input that is very unlikely to be searched for. See
  https://doc.dovecot.org/configuration_manual/fts/tokenization for
  details on how base64 is detected. Only applies when using libfts.
• lmtp: Session IDs are now preserved through proxied connections, so
  LMTP sessions can be tracked. This slightly changes the LMTP session
  ID format by appending ":Tn" (transaction), ":Pn" (proxy connection)
  and ":Rn" (recipient) counters after the session ID prefix.

• Events now have "reason_code" field, which can provide a list of
  reasons why the event is happening. See
  https://doc.dovecot.org/admin_manual/event_reasons/
• New events are added. See https://doc.dovecot.org/admin_manual/list_of_events/
• fts: Added fts_header_excludes and fts_header_includes settings to
  specify which headers to index. See
  https://doc.dovecot.org/settings/plugin/fts-plugin#plugin-fts-setting-fts-header-excludes
  for configuration details.
• fts: Initialize the textcat language detection library only once per
  process. This can reduce CPU usage if fts_languages setting has multiple
  languages listed and service indexer-worker { service_count } isn't 1.
  Only applies when using libfts.
• lib-storage: Reduced CPU usage significantly for some operations that
  accessed lots of emails (e.g. fetching all flags in a folder, SORT, ...)
• lib: DOVECOT_PREREQ() - Add micro version which enables compiling
  external plugins against different versions of Dovecot.
• lmtp: Added new lmtp_verbose_replies setting that makes errors sent to
  the LMTP client much more verbose with details about why exactly
  backend proxy connections or commands are failing.
• submission: Support implicit SASL EXTERNAL with
  submission_client_workarounds=implicit-auth-external. This allows
  automatically logging in when SSL client certificate is present.

• *-login: Statistics were disabled if stats process connection was lost.
• auth: Authentication master user login fails with SCRAM-* SASL mechanisms.
• auth: With auth_cache_verify_password_with_worker=yes, passdb extra
  fields in the auth cache got lost.
• doveadm: Fixed crash if zlib_save_level setting was specified,
  but zlib_save was unset. v2.3.15 regression.
• doveadm: Proxying can panic when flushing print output. v2.3.17
  regression. Fixes:
  Panic: file ioloop.c: line 865 (io_loop_destroy): assertion failed:
  (ioloop == current_ioloop)
• doveadm: stats add --group-by parameter didn't work.
• fts: Using email-address fts tokenizer could result in excessive memory
  usage with garbage email input. This could cause the indexer-worker
  processes to fail due to reaching the VSZ memory size limit.
  Only applies when using libfts.
• imap: A SEARCH command timing out while fts returns indexes may timeout
  returning "NO [SERVERBUG]", while it should return "NO [INUSE]" instead.
• imap: LIST-EXTENDED doesn't return STATUS for all folders. Sending
  LIST .. RETURN (SUBSCRIBED STATUS (...)) did not return STATUS for
  folders that are not subscribed when they have a child folder that is
  subscribed as mandated by IMAP RFCs.
• imapc: Mailbox vsize calculation crashed with
  Panic: file index-mailbox-size.c: line 344 (index_mailbox_vsize_hdr_add_missing):
  assertion failed: (mails_left > 0)
• indexer: If indexer-worker crashes, the request it was processing gets
  stuck in the indexer process. This stops indexing for the folder until
  indexer process is restarted. v2.3.14 regression.
• indexer: Process was slowly leaking memory for each indexing request.
• lib-event: Unnamed events were wrongly filtered out for event/metric
  filters like "event=abc OR something_independent_of_event_name".
• lib-index: 64-bit big endian CPUs handle last_used field in
  dovecot.index.cache wrong.
• lib-ssl-iostream: Fix buggy OpenSSL error handling without assert-crashing.
  If there is no error available, log it as an error instead of crashing.
  The previous fix for this in v2.3.11 was incomplete. Fixes
  Panic: file istream-openssl.c: line 51 (i_stream_ssl_read_real):
  assertion failed: (errno != 0)
• lmtp: Out-of-memory issues can happen when proxying large messages to
  LMTP backend servers that accept the message data too slow.
• master: HAProxy header parsing has read buffer overflow if provided
  header size is invalid. This happens only if inet_listener
  { haproxy=yes } is configured and only if the remote IP address is in
  haproxy_trusted_networks.
• old_stats: Plugin kept increasing memory usage, which became
  noticeable with long-running imap sessions.
• stats: Dynamically adding same metric multiple times causes multiple stats.
• submission-login: Authentication does not accept OAUTH2 token (or
  other very long credentials) because it considers the line to be too long.
• submission-login: Process can crash if HELO is pipelined with an
  invalid domain.
• submission-proxy: Don't use SASL-IR if it would make the AUTH command
  line longer than 512 bytes.
• submission: Service would crash if relay server authentication failed.
• virtual: FTS search in a virtual folder could crash if there are
  duplicate mailbox GUIDs. This mainly happened when user had both INBOX
  and INBOX/INBOX folders and the namespace prefix was INBOX/. Fixes
  Panic: file hash.c: line 252 (hash_table_insert_node):
  assertion failed: (opcode == HASH_TABLE_OP_UPDATE)
• virtual: If mailbox opening fails, the backend mailbox is leaked and
  process crashes when client disconnects. Fixes
  Panic: file mail-user.c: line 232 (mail_user_deinit):
  assertion failed: ((*user)->refcount == 1)
• virtual: Searching headers in virtual folders didn't always use
  full-text search indexes, if fts_enforced=no or body.

• dsync: Add back accidentically removed parameters.
• lib-ssl-iostream: Fix assert-crash when OpenSSL returned syscall error
  without errno.
• master: Dovecot failed to start if ssl_ca was too large.