Below are summaries from the RELEASE_NOTES and HISTORY files.
Incompatibility with Postfix 2.5.3
When a mailbox file is not owned by its recipient, the local and
virtual delivery agents now log a warning and defer delivery.
Specify "strict_mailbox_ownership = no" to ignore such ownership
Bugfix: null-terminate CN comment string after sanitization.
Workaround: avoid "bad address pattern" errors with non-address
patterns in namadr_list_match() calls. File: util/match_ops.c.
Bugfix (introduced 20080207): "cleanup -v" panic because
the new "SMTP reply" request flag did not have a printable
name. File: global/cleanup_strflags.c.
Cleanup: using "Before-queue content filter", RFC3848
information was not added to the headers. Carlos Velasco.
Cleanup: a poorly-implemented integer overflow check for
TCP MSS calculation had the unexpected effect that people
broke Postfix on LP64 systems while attempting to silence
a compiler warning. File: util/vstream_tweak.c.
Paranoia: defer delivery when a mailbox file is not owned
by the recipient. Requested by Sebastian Krahmer, SuSE.
Specify "strict_mailbox_ownership=no" to ignore ownership
discrepancies. Files: local/mailbox.c, virtual/mailbox.c.