Default max file size limit that can be sent and received in cPanel (Exim Mail Server) is 50 MB, and How to Limit Maximum Email Size or Email Attachment ?

To Limit Maximum Email Size do the following,

1. Login to cPanel/WHM as root
2. Under Service Configuration, Click “exim Configuration Manager”
3. Click “Advanced Editor”
4. Scroll down and Click “Add additional Configuration Setting”
5. Find “message_size_limit” in pull-down list and choose
6. Enter limit that you want to change to
7. Scroll to bottom and click “save”