Postfix stable release 2.6.1 fixes one defect in Milter support.
This does not affect Postfix versions 2.5 and earlier.

- Queue file corruption under very specific conditions: (smtpd_milters
  or non_smtpd_milters) enabled, AND delay_warning_time enabled,
  AND mail delivery delays, AND short envelope sender addresses
  (e.g., sendmail command-line submissions with bare usernames as
  the sender, but not bounce messages).

  The queue file would be corrupted when the delay_warning_time
  record was marked as "done" after sending the "your mail is
  delayed" notice.  The defect was introduced with Postfix 2.3, but
  it could not cause corruption before the change dated 20090427.

You can find Postfix version 2.6.1 at the mirrors listed at
http://www.postfix.org/

The same code is also available as Postfix snapshot 2.7-20090519.
Postfix versions 2.5 and earlier are not affected.

        Wietse