SMTP 550 error: "The mail server could not deliver mail to user@domain.tld."

Symptoms

When attempting to send an email to an address on your server, you receive a bounce-back email containing the following message.

The account or domain may not exist; they may be blacklisted, or missing the proper DNS entries.

Description 

This message can occur when the domain's mail routing is set to remote.

Workaround

The domain's mail routing can be changed from "remote" to "local" from cPanel or WHM. The steps to change the mail routing from either location can be found in the following article.

How do I change the email routing of a domain?