cPanel/WHM enforces POP before SMTP authentication by default. With this authentication, if a user authenticates via POP3 then they are allowed to send mail for a period of up to 30 minutes without being prompted for SMTP authentication. If a user hasn’t authenticated via POP3 prior to attempting SMTP, they will be prompted for SMTP authentication.
To always force SMTP authentication regardless of POP authentication, type the following command via SSH as root:
/usr/local/cpanel/bin/tailwatchd --disable=Cpanel::TailWatch::Antirelayd
Incase you want to revert back the change you can simpley run
/usr/local/cpanel/bin/tailwatchd --enable=Cpanel::TailWatch::Antirelayd
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