How to open Exim port 26 on cPanel via SSH
It's no secret that ISP’s will sometimes limit traffic on the common SMTP ports (25 & 587). The result is that clients will constantly contact you because they are unable to send mail. The fix is normally to switch to the alternate port i.e. from port 25 to 587 and vice-versa. This however is only a temporary fix and the client will soon contact you (slightly more frustrated) again with the same problem.