There is an option called 'Default Address' in the Email Management area of your cpanel (http://SITENAME.COM/cpanel OR cpanel.SITENAME.COM). Take care not to use just your domain name as the 'Default Address'. This is wrong and hence all emails to your domain would be rejected by the server.
Default Address is the email account on which you want to receive all unroutables emails to your domain. Unroutable emails to your domain are any email address in your domain name (anything@SITENAME.COM) which you have not set up through cpanel. You should have a valid full email address as 'Default Address' or simply leave it as the account username.
To use any other email address at your domain, you must set these up in your cpanel. If you do not specifically set them up, any emails sent to that address will be considered unroutable and will be delievered to the 'Default Address'. To set up other email addresses, go to the Manage Email Accounts option in cpanel, and add accounts there.
Helpful Links:
cPanel User Documentation
https://documentation.cpanel.net/display/ALD/cPanel+User+Documentation
cPanel Tutorials
https://www.youtube.com/playlist?list=PLZk46idJS6s54hAX8K79_AY8brgBwPtHd
cPanel & WHM Documentation (Dedicated Server Clients & Resellers)
https://documentation.cpanel.net
WebHost Manager - WHM User Guide (Dedicated Server Clients & Resellers)
https://documentation.cpanel.net/pages/viewpage.action?pageId=1507786
