Unless your extremely bogged down-5 minutes should be plenty...
You've been on zimbra since oct 2006 just fine right?
Want to attach your entire postfix log?
Or look for the time differences between the original 'connect from' attempts to the 'timeout after DATA' for smtp102.rog.mail.re2.yahoo.com, smtp01.bis.na.blackberry.com, mercury.golden.net, etc
If you we're hitting max connections you'd see some concurrent connections errors & I would still expect stuff to show up in your queues from time to time during lul's in traffic
(you can do a quick find & replace to pull out your domain names)