Is it possible to completely block inbound email on WHM / cPanel (without blocking sendmail / PHPMail from working)? If so, what config steps are required?
Home »Service Configuration »Service Manager and uncheck
Exim Mail Server. This will disable Exim which is cPanel's SMTP server of choice.
This will however disable outgoing mail too. To workaround this, I suggest using a mail relay.
Another option is to go in to cPanel, and click
Default Address - You can disable the catch-all and set all mail to send back a
:fail: notification from here.