Then how do ISP's get away with blocking port 25. That makes no sense.
I work for an ISP as my day job and we do not allow anything on Port 25.
They block port 25
for their end user customers. I think you may be confused between an ISP blocking port 25 for its end users to send mail, and how servers send mail to other servers.
The ISP
still sends mail to
other servers on port 25. Port 25 is
the Internet SMTP port. If you disable it, you will break mail for your server.
Mail servers always use port 25 to send mail to other servers. That is the Internet standard, and there are no alternate ports. If you block port 25, mail servers that are trying to send mail to your server will only see that port 25 is blocked, and they will not automatically try port 587, because that is not what port 587 is for. Your server will simply not receive any mail because the sending servers will see only that port 25 is blocked, and give up.
Port 587 is the mail
submission port, not the port that mail servers use to talk to each other. It is the port to which a mail client (Outlook, Thunderbird) connects on a server. However, once the server gets the message to be sent from a client, it contacts the remote mail server at the message's destination on port 25, every time.
Blocking port 25 will certainly prevent your customers from sending mail on it, but it will also block all other servers on the Internet from sending mail to your server, effectively breaking mail.
