One way to fix this is as follows:
1. Login to your WHM as root and click on Exim Configuration Editor on your left.
2. Uncheck the box that says: Use callouts to verify the existence of email senders and Save.
3. Go back to the Editor and click the Advanced Editor button.
4. Do a search for sender/callout.
5. When you find it, delete 'callout' and leave only 'sender'. Save.