I've got Zimbra OS running here on ubuntu.

We are seeing random delays (between about 5 mins and 30 mins) when zimbra users send emails to external accounts.

The email headers show;

Received: from ([]) by gse-mta-14.emailfiltering.com with emfmta (version vanilla id 111457464
for oliver.marshall@g2support.com; Thu, 04 Oct 2007 15:55:51 +0100
Received: from localhost (localhost [])
by webmail.g2support.com (Postfix) with ESMTP id 3E00F305D8
for <oliver.marshall@g2support.com>; Thu, 4 Oct 2007 15:33:23 +0100 (BST)

Note the 22 minute delay in the email being passed from Zimbra to the mail filtering service (in this case Email Systems).

Is there anything we can do to see what might be causing this delay ?

It seems that email backup and then get sent out in one hit, as we will receive a whole load of test messages from the zimbra users all in one go with a variety of delay times.