Zimbra errors

[Psyfo.com]

Hi there

Clients are getting an error when trying to send via the web interface and when i checked on the logs it returns the following error:
2009-08-27 15:08:24,931 WARN [btpool0-9186] [name=DDDGGD@example.com;mid=150743;ip=192.168.16.4 2;ua=ZimbraWebClient - [unknown] (Win)/5.0.11_GA_2695.RHEL5_64;] smtp - exception occurred during SendMsg
2009-08-27 15:08:57,129 WARN [btpool0-9198] [name=hehehehe@example.com;mid=96449;ip=192.168.16. 50;ua=ZimbraWebClient - FF3.0 (Linux)/5.0.11_GA_2695.RHEL5_64;] smtp - exception occurred during SendMsg.

su - zimbra

Psyfo@zimbra]$ host `zmhostname`
returns mail.example.com
mail.example.com has address 192.168.0.100

what could be the problem.

thanking you in advanced.