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: 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.