We currently have a 2 machine zimbra setup. One machine is a dedicated spam/filter and the second machine is mailstore/smtp/imap/pop. We noticed that every couple of days the spam filter would stop sending email to the to the mailstore. Also the outgoing smtp on the mailstore also stops sending outbound messages. I issued postqueue -f on the spam filter but that appears to have no effect. Here is the results from postqueue -p

7EB1D27863D 2643 Tue Nov 29 15:43:02 asdf@asdf.com
(delivery temporarily suspended: connect to mail.store.com[69.9.xxx.xxx]: read timeout)
I obviously changed a few of the the names.

Thus far the only way i've been able to correct the problem is by doing a
zmcontrol stop
zmcontrol start
On the mailstore server
Followed by a postqueue -f on the spam filter server.

Currently this email system is not processing a huge amount of email (a few hundred a day at most).