550 5.7.60 SMTP; Client does not have permissions to send as this sender

Problem

When trying to send an email from the Mattermost server through the configured SMTP server, the Mattermost server throws the following error message:

550 5.7.60 SMTP; Client does not have permissions to send as this sender

Solution

This error happens on some SMTP servers if they enforce strict sender rules. What this means is that the sender and reply-to email need to have the same domain name as the SMTP server. So if this error comes up make sure to set the following two configuration settings to the correct email address:

Notification From-Address

Notification Reply-To-Address