I have a very basic Postfix mail server running on my LAN and I'm being asked to change the port clients communicate with the mail server on from port 25 to 587. I'm not sure why I would want to do this but I'm told it's rather common actually. My question is I know my Postfix server will continue to send mail from server to server via port 25 but will now the clients connect to Postfix on port 587 only? Is this how it works? Can someone clarify this for me? I did find and read this: http://www.ietf.org/rfc/rfc4409.txt
No, you just activate port 587 beside port 25 so that client which are unable to reach your server on port 25 can use port 587 instead. So port 25 is still active. To do this, edit the postfix master.cf file and remove the # in front of the "submission" line. Then restart postfix.
