The stable Postfix release is called postfix-2.4.x where 2=major
release number, 4=minor release number, x=patchlevel.  The stable
release never changes except for patches that address bugs or
emergencies. Patches change the patchlevel and the release date.

New features are developed in snapshot releases. These are called
postfix-2.5-yyyymmdd where yyyymmdd is the release date (yyyy=year,
mm=month, dd=day).  Patches are never issued for snapshot releases;
instead, a new snapshot is released.

The mail_release_date configuration parameter (format: yyyymmdd)
specifies the release date of a stable release or snapshot release.

Major changes - critical
------------------------

See RELEASE_NOTES-2.3 if you upgrade from Postfix 2.2 or earlier.

[Incompat 20070122] To take advantage of the new support for BSD
kqueue, Linux epoll, or Solaris /dev/poll, you must restart (not
reload) Postfix after upgrading from Postfix 2.3.

[Incompat 20061209] If you upgrade Postfix without restarting, you
MUST execute "postfix reload", otherwise the queue manager may log
a warnings with:

    warning: connect to transport retry: Connection refused

[Incompat 20061209] The upgrade procedure adds a new "retry" service
to the master.cf file.  If you make the mistake of copying old
Postfix configuration files over the new files, the queue manager
may log warnings with:

    warning: connect to transport retry: Connection refused

To fix your master.cf file, use "postfix upgrade-configuration"
followed by "postfix reload".

Major changes - safety
----------------------

[Incompat 20070222] As a safety measure, Postfix now by default
creates mailbox dotlock files on all systems. This prevents problems
with GNU POP3D which subverts kernel locking by creating a new
mailbox file and deleting the old one.

Major changes - Milter support
------------------------------

[Feature 20070121] The support for Milter header modification
requests was revised.  With minimal change in the on-disk representation,
the code was greatly simplified, and regression tests were updated
to ensure that old errors were not re-introduced.  The queue file
format is entirely backwards compatible with Postfix 2.3.

[Feature 20070116] Support for Milter requests to replace the message
body.  Postfix now implements all the header/body modification
requests that are available with Sendmail 8.13.

[Incompat 20070116] A new field is added to the queue file "size"
record that specifies the message content length.  Postfix 2.3 and
older Postfix 2.4 snapshots will ignore this field, and will report
the message size as it was before the body was replaced.

Major changes - TLS support
---------------------------

[Incompat 20061214] The check_smtpd_policy client sends TLS certificate
attributes (client ccert_subject, ccert_issuer) only after successful
client certificate verification. The reason is that the certification
verification status itself is not available in the policy request.

[Incompat 20061214] The check_smtpd_policy client sends TLS certificate
fingerprint information even when the certificate itself was not
verified.

[Incompat 20061214] The remote SMTP client TLS certificate fingerprint
can be used for access control even when the certificate itself was
not verified.

[Incompat 20061006] The format of SMTP server TLS session cache
lookup keys has changed.  The lookup key now includes the master.cf
service name.

Major changes - performance
---------------------------

[Feature 20070212] Better support for systems that run thousands
of Postfix processes.  Postfix now supports FreeBSD kqueue(2),
Solaris poll(7d) and Linux epoll(4) as more scalable alternatives
to the traditional select(2) system call, and uses poll(2) when
examining a single file descriptor for readability or writability.
These features are supported on sufficiently recent versions of
FreeBSD, NetBSD, OpenBSD, Solaris and Linux; support for other
systems will be added as evidence becomes available that usable
implementations exist.

[Incompat 20070201] Some default settings have been adjusted to
better match contemporary requirements:

- queue_run_delay and minimal_backoff_time were reduced from 1000s
  to 300s so that deliveries are retried earlier after the first
  failure.

- ipc_idle was reduced from 100s to 5s, so that tlsmgr and scache
  clients will more quickly release unused file handles.

[Feature 20061209] Improved worst-case (old and new) queue manager
performance when deferring or bouncing large amounts of mail. Instead
of talking to the bounce or defer service synchronously, this work
is now done in the background by the error or retry service.

[Feature 20061209] Improved worst-case (new) queue manager performance
when delivering multi-recipient mail. The queue manager now proactively
reads recipients from the queue file, instead of waiting for the
slowest deliveries to complete before reading in new recipients.
This introduces two parameters: default_recipient_refill_limit (how
many recipient slots to refill at a time) and
default_recipient_refill_delay (how long to wait between refill
operations). These two parameters act as defaults for optional
per-transport settings.

Major changes - delivery status notifications
---------------------------------------------

[Incompat 20061209] Small changes were made to the default bounce
message templates, to prevent HTML-aware software from hiding or
removing the text "<postmaster>", and producing misleading text.

[Incompat 20060806] Postfix no longer announces its name in delivery
status notifications.  Users believe that Wietse provides a free
help desk service that solves all their email problems.

Major changes - ETRN support
----------------------------

[Feature 20061217] More precise queue flushing with the ETRN,
"postqueue -s site", and "sendmail -qRsite" commands, after
minimization of race conditions.  New per-queue-file flushing with
"postqueue -i queueid" and "sendmail -qIqueueid".

Major changes - small office/home office support
------------------------------------------------

[Incompat 20061217] Postfix no longer requires a domain name. It
uses "localdomain" as the default Internet domain name when no
domain is specified via main.cf or via the machine's hostname.

Major changes - SMTP access control
-----------------------------------

[Incompat 20061214] The check_smtpd_policy client sends TLS certificate
attributes (client ccert_subject, ccert_issuer) only after successful
client certificate verification. The reason is that the certification
verification status itself is not available in the policy request.

[Incompat 20061214] The check_smtpd_policy client sends TLS certificate
fingerprint information even when the certificate itself was not
verified.

[Incompat 20061214] The remote SMTP client TLS certificate fingerprint
can be used for
access control even when the certificate itself was not verified.

[Incompat 20061209] The Postfix installation procedure no longer
updates main.cf with "unknown_local_recipient_reject_code = 450".
Four years after the introduction of mandatory recipient validation,
this transitional tool is no longer neeed.

Major changes - workarounds
---------------------------

[Incompat 20070222] As a safety measure, Postfix now by default
creates mailbox dotlock files on all systems. This prevents problems
with GNU POP3D which subverts kernel locking by creating a new
mailbox file and deleting the old one.

[Feature 20061209] Better interoperability with non-conforming SMTP
servers that reply and disconnect before Postfix has sent the
complete message content.

[Feature 20061209] Better support for queue file systems on file
servers with drifting clocks. Clock skew can be a problem, because
Postfix does not deliver mail until the local clock catches up with
the queue file's last modification time stamp. On systems with
usable futimes() or equivalent (Solaris, *BSD, MacOS, but not Linux),
Postfix now always explicitly sets the queue file last modification
time stamps while creating a queue file.  On systems without usable
futimes() (Linux, and ancient versions of Solaris, SunOS and *BSD)
Postfix keeps using the slower utime() system call to update queue
file time stamps when the file system clock is off with respect to
the local system clock, and logs a warning.

[Feature 20061006] Individual CISCO PIX bug workarounds are now
on/off configurable.  This introduces new parameters: smtp_pix_workarounds
(default: disable_esmtp, delay_dotcrlf) and smtp_pix_workaround_maps
(workarounds indexed by server IP address).  The default settings
are backwards compatible.