Server version: 7.1.2_GA_3268.UBUNTU10_64 UBUNTU10_64 FOSS edition

We are getting some emails returned as undeliverable by our server with the following message,

Code:
host mail.our-zimbra-server-domain.com[xx.xx.xx.xx] said: 451
   4.7.1 Service unavailable - try again later (in reply to end of DATA
   command)
My first guess was that the emails had unusually long attachements that was causing a timeout before the DATA section had completed. However people are reporting that their emails to us are bounced with a size less than 2MB.

I put a watch on the zimbra.log for anything with the word "timeout" in it. There are lots of messages like...
Code:
May 14 17:27:48 mail postfix/smtpd[17009]: timeout after END-OF-MESSAGE from localhost[127.0.0.1]
...and a few like...
Code:
May 14 05:57:31 mail postfix/smtpd[16856]: timeout after DATA (7008 bytes) from unknown[.....
However none of these timeout messages involves one of the IP addresses involved in a sender who has confirmed they were getting their email bounced from us. There doesn't seem to be anything obvious in the logs when it happens, although I am still searching around the date/time of the rejections.

Simply egrepping for "[^0-9]451[^0-9]" doesn't show anything except an unlrelated mention of the number "451".

Our Zimbra server is running on a VSpere 5/SAS/SATA environment on a Dell 2950.

I would appreciate some ideas on what might be causing these timeouts and where they would be reported if not in zimbra.log.