Before cPanel / WHM can authenticate SMTP, it requires a POP authentication. Only after a POP 3 authentication can a user send emails- that too for up to 30 minutes after login.
For forcing authentication through SMTP by default, you just need to run the command as root through SSH:
/ Usr / local / cpanel / bin / tailwatchd-disable = Cpanel:: TailWatch: Antirelayd