Why mail to and from a dynamic DNS Host does not work reliably

No matter what they tell you, mail to and from dynamic DNS Hosts does not work reliably.

At least without TLS it isn't. Here's how it works with TLS: http://www.thalmann.de/postfix-dialup.html

Receiving mail

Why receiving mail does not work reliably:

If you register your dynamic IP in a dynamic DNS provider like dyndns.org, your dyndns.org hostname is assigned your dynamic IP
If your machine loses the internet connectivity, then the hostname will still point to your former IP.
Now somebody else gets your former IP:
- The computer has nothing listening on port 25:
  No problem!
- The computer has a properly setup MTA on port 25:
  Your mail will be bounced! This is bad.
- The computer has an open relay MTA on port 25:
  Your mail will be relayed by this machine.
- The computer has a blackhole MTA.
  Your mail will be lost! This is bad.

Sending mail

Why sending mail does not work reliably:

Nowadays, many providers don't accept mail from dynamically assigned IP pools (like your dialup / DSL IP range). This means you must use the relayhost of your ISP.

© by Ralf Hildebrandt
This document contains links to external information sources that I do neither monitor nor control. I explicitly disclaim any liabilities in respect to external references.
You are getting this document without any guarantees. Any methods shown above are meant as demonstration and may be wrong in some place. You may damage your system if you try to follow my hints and instructions. You do this at your own risk!

This file was last modified 17. Jan 2007 by root