External Ip Doesn't Work Internal

[notevenclose]

Hi!
I have a possibly non-xmail-related problem, but maybe someone can help me anyhow.

I'll explain the situation: We have a hosted mailserver on the internets and an internal mailserver inhouse (debian, PPC, xmail 1.2.5). All the external POP-retrieving stuff works, as well as relaying emails over our hosted mailserver.

What doesn't work is the following: Some guys here want to read emails at home or on holidays (crazy, I know).
So I have forwarded the ports 25 and 110 (we have a static IP here). So people take the external IP for POP and SMTP.
The part that doesn't work is when someone takes his notebook in-house it doesn't work anymore, it works again if the person makes the internal IP of our xmail-server for POP/SMTP.

I don't know why this happens, ping and telnet on 25 and 110 works from in-house on the external IP.

Am I trying something impossible? I'm not a network-pro, neither a xmail-pro.

<- a ninja, cause they're awesome (like you, if you can help me)

[hschneider]

Your router uses NAT (Network Address Translation) to route internal addresses thru its external interface. So your router can route packets thru your external IP, but not in a loop back to itself. This works for ICMP packets used by Ping, but not for regular data packets.

You can easily probe this from the inside or outside your LAN with the tracert command on Windows or traceroute on (nearly) any other OS:

tracert exterrnal_ip

Solution: Give your mobile customers a name instead of an IP and configure your LAN's DNS to resolve this name to the internal, local IP.