1) SMTP AUTH ON
If you have "SMTP AUTH ON" (assp WHM interface) , your customers MUST
authenticate ALSO to send email (not only to receive). If your customer does not
authenticate he can't send.
With smtp auth ON your client must use following settings to send email
without errors
smtp : mail.clientdomaim.com
plus smtp authentication ON
2) SMTP AUTH OFF
If you have "smtp auth OFF" (ASSP WHM interface) , your client should
authenticate only POP3 email .
Your customemers may use only pop3
authentication . Your users authenticates using pop3 , and cPanel antirelayd
stores on /etc/relayhosts ips allowed to send email . If you have "smtp
auth OFF" and your client can't send
email be sure you have "antirelayd cpanel service" running. If you want
check if antirelayd is running ...