Settings MX records manually can be useful if you want to host your site on one server but handle email via another server or hosting company. If you have access to WHM, you can usually set MX records yourself. If you do not, submit a support ticket to your hosting provider.
Adding and Settings MX Records in WHM (Web Host Manager)
domain name | TTL | Record Type | Priority | MX Record
example.com. | 14400 | MX | 0 | mx1.example.com. Post any questions or comments below 
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One more step
If the email for a domain is handled by an external server, shouldn't the domain also be removed from /etc/localdomains and added to /etc/remotedomains ?
This is easy if one is running a VPS, but if it is just a reseller account, one has to ask the hosting company to do it.
A word or two about how to make the changes using command line would add to the value of this tutorial.