I'm having the same problem. We needed to allow a normally-blocked attachment from being blocked yesterday, then again this morning, and both times, Zimbra stopped relaying mail. I tracked down this being the culprit in both instances, but here's the one from this morning:
/opt/zimbra/log/zmconfigd.log shows:
2011-03-10 08:57:04,526 zmconfigd WARNING [26677-MainThread] Var zimbraMtaBlockedExtension changed from 'asd bat chm cmd com dll do exe hlp hta
js jse lnk mov ocx pif reg rm scr shb shm shs vbe vbs vbx vxd wav wmf wsf wsh xl' -> 'asd bat chm cmd com dll do exe hlp hta
js jse lnk ocx pif reg rm scr shb shm shs vbe vbs vbx vxd wmf wsf wsh xl'
Followed by /var/log/maillog showing:
Mar 10 08:57:16 myhostname postfix/postfix-script[15566]: refreshing the Postfix mail system
And seconds later in /var/log/maillog:
Mar 10 08:57:18 myhostname postfix/smtpd[15571]: connect from some.hostname.net[1.2.3.4]
Mar 10 08:57:20 myhostname postfix/smtpd[15571]: 8A8C2B58001: client=some.hostname.net[1.2.3.4], sasl_method=LOGIN, sasl_username=ausername@mydomain.com
Mar 10 08:57:20 myhostname postfix/cleanup[15576]: 8A8C2B58001: message-id=<C99E4383.6829%ausername@mydomain.com>
Mar 10 08:57:20 myhostname postfix/qmgr[15577]: 8A8C2B58001: from=<ausername@mydomain.com>, size=3652, nrcpt=1 (queue active)
Mar 10 08:57:20 myhostname postfix/smtp[15578]: connect to 127.0.0.1[127.0.0.1]:10024: Connection refused
Mar 10 08:57:20 myhostname postfix/smtp[15578]: 8A8C2B58001: to=<someone@anotherdomain.com>, relay=none, delay=0.42, delays=0.41/0/0/0, dsn=4.4.1, status=deferred (connect to 127.0.0.1[127.0.0.1]:10024: Connection refused)
This went on for over an hour before we realized messages weren't being delivered. Another administrator restarted Zimbra before I was able to check any other commands. Restarting Zimbra fortunate did fix the problem, but I'm assuming any change to zimbraMtaBlockedExtension will continue to break the MTA.
Running ZCS version zcs-7.0.0_GA_3077.RHEL5.20110127201734 on RHEL 5.1 32-bit.