Symptoms
A domain is unable to send or receive mail when using third party name servers.
Description
When your DNS provider is a remote host such as Cloudflare, if the appropriate mail records have not been configured, mail will not be delivered as intended.
Workaround
To address this, you'll need to create an MX record for your domain at the DNS host (example: mail.domain.com) and ensure that there is a corresponding A record pointing the MX record to your server. Finally, you will want to make sure that the domain name's routing is correctly set in the user's cPanel -> Email Routing to "Local Exchange".