SMTP - timeout after DATA

[Eric]

I have Zimbra 4.04 installed on Fedora (FC5).
About once a day the SMTP starts timing our with messages like the following:
Jan 3 14:01:27 mail postfix/smtpd[16769]: timeout after DATA from unknown[192.168.0.111]
The behavior is rather erratic. Some mail does go through and other does not (mostly not). I may be able to send to a specific address and then a minute later I get a timeout.
Restarting Postfix does not solve the problem. Neither does restarting Zimbra. However, a full system reboot works every time, until the next time...
Googling brings the following Postfix reference: http://www.postfix.org/faq.html#timeouts
However since our problem is not consistent and the system works fine most of the time, I do not think it applies to us.
Ideas? Thanks.

[phoenix]

Firewall & SElinux disabled? Hardware problem, failing cable? Intermittent DNS problem? Any of those applicable? I'd suggest looking at your DNS server for problems.

[Eric]

Originally Posted by phoenix

Firewall & SElinux disabled? Hardware problem, failing cable?

Firewall and SElinux disabled. No hardware problem. The system shows 1-3% CPU during fails and responds well with IMAP and web interface.
DNS is a good guess though I'm not sure what too look for. The inconsistency is rather baffling.

[phoenix]

Single DNS server? Is it on your LAN or external? Do you get other connectivity problems during the day that might be related to this? What happens if you add another DNS server?

[Eric]

Originally Posted by phoenix

Single DNS server? Is it on your LAN or external? Do you get other connectivity problems during the day that might be related to this? What happens if you add another DNS server?

Now that I think of it I'm not sure DNS is the issue. timeout after DATA means that the handshake with the server was OK and that the timeout occurred after a successful connection. It seems more like a data transfer problem. I will start checking for congestion and other network stuff. What is strange is that I have been using different server until lately and had no such problem