WHM Email server: receiving issue, manualmx entry

Recently I was asked to setup a development / staging server, I was given one WHM installed Ubuntu server, I have setup one sub domain and its sub domains to replicate the live server different applications and API.

Accidentally email stopped working at live server, I checked all settings, also validated the email delivery reports and all options checked, but email was not receiving at any server. Strange thing was that I was working at dev server, and email stopped for live server.

When I took log of exim from dev server I saw all emails was coming to dev server, and it is relaying to live server, quite strange.

By using live server mail troubleshooter I saw email is pointing to dev server, I checked few things like /etc/hosts and other DNS settings, nothing was wrong there.

Than I saw that it is saying manualmx, and pointing to dev server, while getting more information about the manualmx, it came to my knowledge that we can override the MX record by using this file, as soon I removed the entry, everything start working as previously.