Sendmail: Deferred: Name server: xxx.xxx.xxx: host name lookup failure

[Bas Hendriks]

Hello,

I have a problem with one of my servers,
Suddenly i am getting a lot of "Deferred: Name server: xxx.xxx.xxx: host 
name lookup failure" while DNS resolving on that box is ok(from the 
command line de backend servers do resolve and are reachable). The mail 
stays in the mailqueue.

This is a mailscanning server (Mailscanner/Spamassasin/Clamav), witch 
functions as primary mx for several domains, after receiving  mail it 
forwards  this mail to backend servers(Using mailertable: domain.com 
smtp:backendserver-n.xxxx.xx).

Any pointers on how to debug this would be appreciated!

Best regards,
Bas Hendriks

--
extra info:
[bas at mailscanner bas]$uname -a
FreeBSD mailscanner.xxxxxx.xxx 6.0-RELEASE FreeBSD 6.0-RELEASE #0: Thu Nov  3 09:36:13 UTC 2005     root at x64.samsco.home:/usr/obj/usr/src/sys/GENERIC  i386
[bas at mailscanner bas]$telnet 0 25
Trying 0.0.0.0...
Connected to 0.
Escape character is '^]'.
220 mailscanner.xxxxxx.xxx ESMTP Sendmail 8.13.4/8.13.4; Mon, 17 Jul 2006 16:19:58 +0200 (CEST)