When that is happening, if you look at the delivery logs, you may see an error like this:
Nov 18 11:07:33 bolt14a postfix/smtp[22329]: 3jhxH02qjtz29yWZ: to=<somebody@example.com>,
relay=mail.example.com[192.168.1.1]:25, delay=33, delays=33/0/0.18/600, dsn=4.4.2, status=deferred
(conversation with mail.example.com[192.168.1.1] timed out while sending message body)
Have your email server administrator check your mail server logs to make sure a spam or virus filter isn't dropping the connection. This is the most common cause of timeouts that we see. If nothing is found in the mail server logs, have check them check the firewall logs.
While we are not Cisco experts, but we have had customers solve this issue with their Cisco firewalls in two different ways:
no fixup protocol smtp 25
Our service uses a standard MTU (Maximum Transmission Unit) setting of 1500. If any device on your network or your internet provider's network uses a setting lower than 1500, it can cause time outs during message delivery. To change the MTU setting on your firewall, router and mail server, please refer to the documentation for that device.
If none of the above solutions solve your issue, this may be the cause. In this case, we can assign your domain to our low MTU servers which will sometimes solve the issue. Contact our team to have us try this option for you.
If this solves your issue, it could mean that the time-outs might be caused by poor network connectivity which are beyond your control (for example, if there's a bottleneck with an upstream network provider between your provider and ours).
If a lower MTU setting works better in some situations, why not use it for all domains? We would, except it is also less efficient and would use more bandwidth. It's also slightly slower and would slow email transmission for everyone. That said, if your email delivery has frequent time outs, then it could actually speed up email transmission considerably.