Update Postfix to version 2.5.1 (ok martti).  Major changes over 2.4.x are:

- TLS (SSL) support was streamlined further, and provides a new security level
  based on certificate fingerprints instead of CA signatures. See TLS_README
  for details.

- Milter support was updated from the Sendmail 8.13 feature set and now
  includes most of the features that were introduced with Sendmail 8.14. See
  MILTER_README for details.

- Stress-adaptive configuration was introduced. This allows the Postfix SMTP
  server to temporarily adjust its rules under conditions of overload, such as
  a malware attack or backscatter flood. See STRESS_README for details.
  [pkgsrc: this obsoletes the "postfix-stress" option which provided the same
  functionality via a distribution patch]

- The queue manager scheduler was refined. It now provides per-transport
  scheduling controls and allows for adjustment of the sensitivity to mail
  delivery (non-)errors. See SCHEDULER_README.

- Security was improved by introducing a Postfix-owned data_directory for
  storage of randomness, caches and other non-queue data. This change avoids
  future security loopholes due to untrusted data sitting in root-owned files
  or in root-owned directories. Writes to legacy files in root-owned
  directories are automatically redirected to files in the new data_directory.

No functionality has been removed, but it is a good idea to review the
RELEASE_NOTES file for the usual minor incompatibilities or limitations.

(ghen)