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4 reasons for “unknown mail transport error” in Postfix servers

Emails are vital for online businesses. That’s why email errors are a nuisance for server owners.

At Bobcares, we often resolve many such email errors as a part of our Technical Support Services for web hosting companies.

Today, we’ll see what causes “unknown mail transport error” in servers and how we fix them.

What causes “unknown mail transport error” in servers

Postfix is a commonly used mail server in Linux servers. Many control panels like Plesk offers Postfix mail server as part of its installation.

Postfix can also be integrated with third party mail server providers such as SendGrid. But at times, Postfix can throw errors during mail delivery.

One such issue is when server owners report of mail delivery errors. We examine the mail logs and notice a lot of undelivered outgoing emails in the mail queue.

The mail logs at /var/log/maillog often show these errors:

postfix/error[12027]: 541B4649412: to=<>, relay=none, delay=113711, delays=193632/1/0/0.01, status=deferred (unknown mail transport error)


The major causes for this error are:

1. Postfix configuration issues

The main configuration file of Postfix is /etc/postfix/ This file contains the default Postfix settings such as transport settings, network, mail server IP and so on.

In cases of external mail server or other special requirement, custom settings have to be done in this configuration file. This include adding IPs that can relay through the server.

Any wrong entries in this configuration file can lead to errors in email delivery, such as  “unknown mail transport error”. Software updates or patches can also corrupt Postfix installation and cause errors.

2. File permissions

The mail queue, configuration file, filters and mail folders, all these need to have proper ownership and permissions for mail delivery to work fine.

Usernames and passwords are saved in the file /etc/postfix/sasl_passwd. This file should be readable by Postfix service and need to have proper ownership and permissions.

After adding entries to the password file, it should be mapped to a database file in postfix. The database and other related files should have the right access privileges.

3. Missing aliases

Email aliases usually refers to forwarders to various email accounts. It can happen that sometimes the aliases contain non-existent or invalid email accounts, giving out errors in email delivery.

4. Third party integration

Postfix can be integrated with third party mail servers like SendGrid, Mandrill, etc. For mails to relay through their servers, custom configuration has to be done in Postfix.

In some installations of Postfix, only alphanumeric passwords are allowed. If the SendGrid password is not in that format or there are any configuration mistakes, mail server will throw errors.

How to fix “unknown mail transport error” in Postfix servers

1. Fix configuration errors

The major reason for “unknown mail transport error” is the error in configuration file. We figure out the erroneous entries and correct them.

In cases where the Postfix configuration is corrupt, we rebuild the configuration files and repair the Postfix installation and confirm that mails are working fine.

2. Examine email aliases

If the error is specific to only specific email addresses or aliases, we examine those aliases and figure out the missing accounts. We update these aliases for the mails to work fine.

3. Permission and ownership updates

We cross-check the permissions and ownership of the related files and folders and update them so that Postfix service can access them and deliver mails without errors.

4. Reinstall Postfix

In cases where the error occurred after any software or package updates, we pinpoint the module or patch that caused the issue. We reinstall or reconfigure those to ensure successful mail functioning.

In the servers we manage, we conduct periodic server audits to confirm the success of software updates and functioning of services. For the upgrades we perform, we take actions to avoid such issues.


Email errors can be vexing for server owners. Today, we saw how our Outsourced Tech Support team easily fix the error ‘unknown mail transport error’ for our customers.



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Bobcares is a server management company that helps businesses deliver uninterrupted and secure online services. Our engineers manage close to 51,500 servers that include virtualized servers, cloud infrastructure, physical server clusters, and more.