I'm trying to sent a test mail using the "Mail templates" section in the CMS. OctoberCMS says that the test mail has been sent, but I'm not receiving it. The credentials under mail configuration appear to be correct as OctoberCMS throws an error that they are incorrect otherwise. To be clear: the email is not in my spam box.

How do I fix this issue?

This could be for any number of reasons. First place to check would be the logs of your mail sender/provider to see if the message is actually reaching them

Found it! Had nothing to do with OctoberCMS. Had to turn off the "Use this server to handle my emails." checkbox under MX Records in DirectAdmin. Thanks for the reply, @daftspunk!