Sometimes you purchase a domain and hosting at truehost with the email of hosting your emails but hosting the website in another.
Normally, once you change the A records of a domain, the mx records automatically changes to where the A records are.
So how do you host your emails at truehost and the site elsewhere?
Login to whm> edit dns zone and select the domain name you want to host
Login to cpanel of your domain>domains>zone Editor
Under the A records, you will see the A records created with the Truehost IP address
Now create the mx records with the A records you have created.