"User Unknown" errors related to DNS issues?

Mon Dec 13 22:38:53 UTC 2004

Hi all,

I see a periodic problem with some email servers that report "User
Unknown" for valid users.  The most recognizable problematic host is
hotmail.com, but there usually are others mixed in (today's additional
problematic hosts included msn.com, verizon.net, and incentre.net).

Here is one of the errors:

  Dec 13 11:20:55 mail sm-msp-queue[5360]: iBDGFPc0004630:
  to=<frank-****@msn.com>, delay=00:05:30, xdelay=00:00:00,
  mailer=esmtp, pri=470333, relay=mx1.hotmail.com. [64.4.50.99],
  dsn=5.7.1, stat=User unknown

I found this post on Neohapsis that suggests the problem is DNS/firewall
related:

  http://archives.neohapsis.com/archives/postfix/2004-03/1774.html

What I can't understand is why this would be periodic and what, if
anything, I can do to validate the authenticity of the errors?

Do others see this same issue?

-Jim P.