Error "timed out while performing the EHLO handshake"

[wftl]

We have a brand new (a month or so) Zimbra installation running on a single server. The DNS is offsite and the server hosts email for a handful of different domains. For the most part, everything works perfectly and email does what email is expected to do. However, with a few domains, all messages are 'deferred' with the following message.

status=deferred (conversation with mx2.somedomain.com[XX.161.132.XXX] timed out while performing the EHLO handshake)

If I send an email to these domains from another server (on another domain) running plain old Postfix (no Zimbra) also with multiple domains, it works perfectly. I'm at a loss to see what the real issue is here. Any suggestions are most welcome.

-- Marcel

[phoenix]

Is your zimbra server vitualised in xen, by any chance?

[wftl]

Hello phoenix,

No, this server is running on its own hardware with Red Hat Enterprise Linux Server release 5.