For years, many hosting providers have been blocking port 25 in an effort to combat spam. While this can help avoid spam abuse on the network, it does cause problems for users who are sending valid email from their servers. By default, all mail servers use port 25 when it comes to relaying messages to other servers. Blocking this port prevents your server from being able to connect and deliver messages to any remote locations. If your provider has blocked port 25, only incoming email and local mail delivery would be operational. However, there are still options available to allow your server to send email.
First, it is important to note that only changing the outgoing SMTP port is not an option. While cPanel can alter the SMTP port for incoming messages, other remote servers are not guaranteed to be listening for incoming messages on those same ports. Even if these ports are open for some destinations, the remote servers may not be configured to receive messages relayed in this way over those ports. This means messages would need to be sent over port 25 to ensure that they are received properly by the remote servers.
Next, many providers will inform you about potential blocks in their documentation somewhere. If you do not see a block in either location, you can test this for yourself via SSH on the server or the Terminal in WHM. This test would use either the "nc" (netcat) or "telnet" command:
nc mail.remote-server.tld 25 telnet mail.remote-server.tld 25
If you are using our cPanel hosting package, you can change the SMTP port from TCP/25 to TCP/501 on your SMTP setting.