Why the ‘unknown mail transport error’ Occurs in Postfix Server? And How to Fix it?

Postfix is used commonly in Linux servers as the default mail server. Also in control panels like Plesk, you’re going to find Postfix installed in it. Just like other services, it is susceptible to errors. There are many errors users and admins can come across. And one of them is the ‘unknown mail transport error’. Its the server owners who get this error when sending emails. If you navigate into the mail logs at /var/log/maillog:

In this article, learn why this error occurs and some of the ways to fix it.

4 Reasons Behind the unknown mail transport error

There are four reasons why this error may occur. Those are:

• Configuration issues in Postfix

Postfix has a configuration file at /etc/postfix/main.cf. It contains a wide range of settings for transport settings, network, mail server IP, among others.

If the mail server you’re sending emails to is external, you need to apply custom settings in the configuration file. Among other things, you’d have to add IP addresses that can relay through the server.

When making the changes, you need to be careful. Any incorrect information in this file will lead to ‘unknown mail transport error.’ In some cases, software updates and patching also make changes to the configuration files. This leads to the transport error in some cases.

• Incorrect file permissions

For an email to be delivered successfully, its important that the correct ownership and permission are set for mail queue, filters, configuration file, and mail folders.

You can access usernames and their respective passwords at /etc/postfix/sasl_passwd. You need to make sure that you’ve set up adequate ownership and permissions that the Postfix services needs. Also, you’d have to add entries to the password file and map them to a database file in postfix. The database, too, should have required privileges.

• Aliases missing

Email aliases are essentially the forwards to other email accounts. In some cases, the alias may contain invalid email accounts or accounts that don’t exist. So you’re likely to face errors in email delivery.

• Incorrect third party integration

You can integrate Postfix with third-party services like SendGrid. But for the emails to be successfully relay through their servers, you must apply custom configuration correctly.

In some versions of Postfix, you’re limited to using alphanumeric passwords. In case the SendGrid doesn’t have the same format, you’re likely to run into this error.

Fixing the Error

There are a few ways you can fix the error. And it starts with recognizing the core issue. Here are the solutions:

• Fix configuration errors

The primary reason for this error is misconfiguration. Therefore, you need to fix the configuration errors at the config file. Figure out the erroneous entries and fix them.

If the configuration file is corrupted, you need to rebuild it and repair Postfix installation. This should fix the error.

• Check the email aliases

If email aliases are missing or email accounts doesn’t exist, then you need to update the alises to fix the issue.

• Update permission and ownership

To ensure Postfix service is able to access required files and folder, you need to update the permission and ownership details.

• Reinstall Postfix

Lastly, you can reinstall Postfix if the error resulted from a software or package update. Or, you can also update/restore specific module or patch that might be causing the error.

So that’s how you deal with the ‘unknown mail transport error’. Its a relay error that affects mail delivery. For more assistance, contact the support team.