Hello,
First you will have to point your domain MX record to server
"B" and then setup the "Remote Mail Exchanger" for that domain on server
"A" you can do it through following steps
Login your server
"A" through In WHM > DNS Functions > Edit DNS Zone area, select the domain in question and click the "Edit" button. In the new screen that loads, scroll to the bottom and you will see the following:
Email Routing for (The domain is owned by username.)
Automatically Detect Configuration: Auto (recommended) more »
Local Mail Exchanger more »
Backup Mail Exchanger more »
Remote Mail Exchanger more »
Current setting is shown in bold.
Select the "Remote Mail Exchanger" radio button and click the "Save" button,
Once it's done create the cPanel Account on your server
"B" and create your mail account through cPanel on server
"B"