Error: unable to connect to mx servers mailgun

What's Causing This Error?

This error occurs when Mailgun cannot establish a connection with the mail server of a recipient's email address. The Mailgun community has identified that the most common reason is when the recipient's email service provider has received many emails from your IP over a short period.

Solution - Here's How To Resolve It

Mailgun recommends waiting for one hour (until the previously sent emails have been processed by the recipient's mail provider) before trying again.