By default cPanel / WHM requires POP before SMTP authentication. With this authentication, if a user is authenticated via POP3, then they are allowed to send e-mail for a period of up to 30 minutes without being prompted for SMTP authentication. If a user is authenticated via POP3 before SMTP attempt, which prompted for SMTP authentication.
To force authenticate via SMTP by default, simply run the command as root via SSH:
/ Usr / local / cpanel / bin / tailwatchd-disable = Cpanel:: TailWatch: Antirelayd