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How do I allow external users to relay mail through my server?

Question Background:
How I can setup my GMS server to allow outside users to relay their mail through my server. I only want to allow users from my domain to do this, I don't want to open this up to everyone on the Internet.

I have some sales people that travel and they need to both send and receive mail on the road. I have set them up to POP their mail but they cannot send mail back through my system. What can I do?

Answer:
There are 3 options that you should be looking at.

1. Local IP - It is possible to allow a single IP address or a range of IP addresses to be classed as local. Any user connecting from within this range is then allowed to relay. This option is configured under the Security > Local IP. It should only really be used with static IP addresses
2. Authenticated SMTP - Enable "Allow AUTH" from the Incoming > ESMTP page. Your remote users will then have to authenticate to the SMTP service when sending external messages through your server. The drawback is that this is only supported by some mail clients.
3. POP-before-SMTP and IMAP-before-SMTP - this requires that you have a GMS Anti-Spam key, but has the advantage of being supported by all mail clients. These options are enabled and controlled under the Anti-Spam > Connect > Authenticate section of the GMS Administrative interface