Our usually well-behaved Zimbra installation has popped up with a problem twice in the last week.

Users complain about SMTP errors when sending from Thuderbird, and when I check I find that the SMTP conversations are hanging after the mail from line
For example

220 mailX.viz.com ESMTP Postfix
HELO test.com
250 mailX.viz.com
MAIL FROM:<myemailaddress@viz.com>

and it just hangs.

All that is logged in the zimbra.log file is:
Feb 2 13:48:48 mail3 postfix/smtpd[9039]: connect from unknown[]

Restarting zimbra clears the problem (but cheeses off my users).

Any ideas?

Release 6.0.8_GA_2661.RHEL5_64_20100824100342 RHEL5_64 NETWORK edition. on RHEL 5

I know we're a couple of revs behind, but things have been stable on 6.0.8 for quite a while before this popped up.