How does an MX relay work, as in, how is my SMTP server supposed to use it?

Question

I'm trying to set up a Postfix server (any other software would be ok for me as well). I don't have a static IP, and that can apparently be quite a problem for e-mail purposes: I managed to "send" mails with the default configuration, but the dynamic IP is immediately rejected by any target I try to mail.

Now, DynDNS (which I already use for the domain) offers these things called gateway Mail Exchangers: mx1.mailhop.org, mx2.mailhop.org; I take it that these are basically fixed-IP proxies that solve exactly this kind of dynamic-IP trouble (plus some spam-protection stuff).

Unfortunately, I have absolutely no clue as to how I'm supposed to set up Postfix to actually use these things. MX appears to be some kind of "record" that tells other SMTP servers where to find mine, but I haven't found anything about this in the Dyn-account settings. And the postfix documentation has a lot to say about various relays, but is this like a firewall, a LAN, or what?

I would be greatful about any clarification how it all works, ideally /etc/postfix/main.cf instructions.

Answer 1

First of all using a dynamic IP in any production environment is a terrible idea and an administrative nightmare. Secondly you should look into hiring a trained network administrator.

However to answer your question, Postfix, or any SMTP server really, doesn't have any direct interaction with the MX records. They are nothing more than DNS records that tell mail servers how to reach a destination (IE DNS A records). An example: User Joe wants to send an email to user@example.com so the local mail server makes a DNS query and comes back with mx.example.com which uses the A record mail.example.com as the destination.

Understand this is an extremely basic overview of how it works and the actual process is much more involved, but this is how I would explain it to an intern or student. Mail relays are a totally separate deal that mail servers use to decide how and when to send the mail itself and isn't relevant to your question.