I have a web server setup with postfix which is relaying email to a zimbra server. This working fine however I have attempted to send a few thousand emails and now the connection from postfix to zimbra is timing out. All of the emails have been deferred on the postfix queue.

If I try to send individual emails from postfix to zimbra it works fine. But if I try to flush the postfix queue all of the emails time out. In mail.log the emails look like this:

postfix/error[2494]: 32B0950C04: to=, relay=none, delay=19431, delays=19402/29/0/0.01, dsn=4.4.1, status=deferred (delivery temporarily suspended: connect to mail.server.com[123.45.678.91]:25: Connection timed out)

I have also noticed that in the above message it says "relay=none" for these emails that are failing. But the emails that do send say "relay=domainname.com".

How I can resolve this, by sending the emails in the queue and avoiding this from happening again?