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As of September 12, 2018 you can setup relayhosts as admin by using the mailcow UI.

This is useful if you want to relay outgoing emails for a specific domain to a third-party spam filter or a service like Mailgun or Sendgrid. This is also known as a smarthost.

Add a new relayhost

Go to the Routing tab of the Configuration and Details section of the admin UI. Here you will see a list of relayhosts currently setup.

Scroll to the Add sender-dependent transport section.

Under Host, add the host you want to relay to.
Example: if you want to use Mailgun to send emails instead of your server IP, enter

If the relay host requires a username and password to authenticate, enter them in the respective fields.
Keep in mind the credentials will be stored in plain text.

Test a relayhost

To test that connectivity to the host works, click on Test from the list of relayhosts and enter a From: address. Then, run the test.

You will then see the results of the SMTP transmission. If all went well, you should see SERVER -> CLIENT: 250 2.0.0 Ok: queued as A093B401D4 as one of the last lines.

If not, review the error provided and resolve it.

Note: Some hosts, especially those who do not require authentication, will deny connections from servers that have not been added to their system beforehand. Make sure you read the documentation of the relayhost to make sure you've added your domain and/or the server IP to their system.

Tip: You can change the default test To: address the test uses from to any email address you choose by modifying the $RELAY_TO variable on the file under /opt/mailcow-dockerized/data/web/inc
This way you can check that the relay worked by checking the destination mailbox.

Set the relayhost for a domain

Go to the Domains tab of the Mail setup section of the admin UI.

Edit the desired domain.

Select the newly added host on the Sender-dependent transports dropdown and save changes.

Send an email from a mailbox on that domain and you should see postfix handing the message over to the relayhost in the logs.