I've seen numerous posts on people experiencing problems with their email sitting in deferred queues, I am pretty much getting this error on most emails with a PDF attachment.

Below is the detail of the zimbra.log file:

Mar 26 17:31:05 zimbra postfix/lmtp[20062]: D6914A047DC: to=<user@domain.com>, relay=zimbra.domain.com[66.253.123.229], delay=6, status=deferred (host zimbra.domain.com[66.253.123.229] said: 451 4.0.0 Temporary message delivery failure try again (in reply to end of DATA command))
Mar 26 17:31:06 zimbra postfix/lmtp[20104]: F3313A047DB: to=<user@domain.com>, relay=zimbra.domain.com[66.253.123.229], delay=5, status=deferred (host zimbra.domain.com[66.253.123.229] said: 451 4.0.0 Temporary message delivery failure try again (in reply to end of DATA command))

Probably about 5% of the emails get stuck in the deferred queue, and I have to manually requeue them, and usually after a couple of times they will get sent. I now have several that no matter how many times I requeue the emails they will not get delivered and end back up in the deferred queue. All messages ever in the deferred queue have the "in reply to end of DATA command" as the failure reason.

Is there a way to get Zimbra to automatically retry it's deferred queue, rather than just letting the emails accrue there forever, without restarting the server or taking it down?

Also what can I do to actually keep this from happening? Kinda seems silly this permanent failure is stated as being 'temporary'. Is there some set of required libs or .so files not getting installed by the Zimbra installation program?

I am running 4.5.3 Network edition on Ubuntu 6.01 Dapper LTS. Recent, clean install.