SMTP outbound blocked – AT&T Community Support

SMTP outbound blocked – AT&T Community Support

Re: SMTP outbound blocked

‎11-13-2010 12:39:20 AM

Hah…

Sure, you can have your mailserver bind to whatever port(s) you’d like. But it does no good.

Port 25 is what all mailservers use to communicate across the ‘net. Listening on port 26 does you no good as no one else is going to send you mail by that port. Sending mail out port 26 won’t get get anywhere, because no one else listens/sends on 26. 26 isn’t even assigned to a protocol per IANA.

So if port 25 outbound is blocked by my ISP, my mailserver (which is my MTA/MX) can not send mail to the rest of the world. Sending or listening on port 26 does no good because no one else uses it. Your mailserver works because… wait for it… you’re using port 25. Same as mine… and everyone else’s.

Now sure, if I was using some other MX/MTA as a smart host to relay all my mail off, and I had control of that server too, I could configure it and my mailserver to talk to each other along non-standard ports. But I don’t need to, nor would I really want to unless I didn’t have control of my PTR record or something. In THAT case, I’d use a standard SMTP TLS port like 465 or 587 and secure that communication.

Regardless, that upstream MX/MTA relay would still need to be able to send/receive on port 25 itself, so it could talk to the rest of the world’s mailservers.

Does that “It won’t work” post make more sense now?

See… I couldn’t send mail out because of the 25 block… sending out 26 (or any other non-SMTP port) wouldn’t work, and by standard MXs don’t communicate to each other across the Interenet over TLS. That’s what we mean by “it won’t work.”

via SMTP outbound blocked – AT&T Community Support.

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