From Arch Hosting Wiki
If you're having trouble sending out emails, there are tools in your cPanel that can assist you with this.
The first tool is "Track Delivery", as seen here:
On this page, you should click the "Show All" button.
After that, it will list all of your emails and the associated status message.
The next tool is the Email Deliverability page in your cPanel.
On this page, click "Manage" next to the domain encountering issues and you will be able to see a list of errors and suggested fixes.