Occasionally we run into an issue with a destination mail server that is slow to receive email and will even cause time outs that cause noticeable delays on email delivery.
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: 3jhxH02qjtz29yWZ: to=<email@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)
Issues like this can be hard to solve as it usually indicates that something on the receiving end is dropping the connection. If you have access to your mail server's logs, that might provide some clues as to why it's dropping the connection. Here are some possible solutions:
1. Check your mail server and firewall logs.
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.
2. Check if you use a Cisco firewall.
If you use a Cisco brand firewall, make sure the "MailGuard" and "fixup" features are turned off on port 25.**
While we are not Cisco experts, but we have had customers solve this issue with their Cisco firewalls in two different ways:
- According to one customer with a PIX/ASA firewall, there is a "MailGuard" feature that inspects SMTP traffic and restricts SMTP commands to a minimal set. Disabling this feature may resolve the time out issue.
- According to another customer with a the PIX/ASA firewall, the solution was to run the following from the Cisco command line (and "save" afterwards):
no fixup protocol smtp 25
We appreciate the feedback from our customers. If you discover other solutions that help with Cisco firewalls, we would love to know about them.
3. Check your firewall, router and mail server's MTU setting.
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.
4. Try switching to our low MTU servers.
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.