Ok, I think I understand your setup now.
Minor thing before I forget...
Ip addresses x.x.x.0 and x.x.x.255 are not "real" IPs. 0 identifies the network, and 255 is a broadcast address. The reason I bring this up is you may want to reduce your LAT to 254, the last usable address.
Internal mail is working, so your Exchange box is doing what it should for users.
Here are the dumb questions again...
How is your smtp authentication/relay set up? My relay security settings stopped incoming mail for a while. I have posted the Ex2k settings that I used on the Server Publishing section, but I'm not sure how to configure it for 5.5.
Did the Ex5.5 server work before? What I am trying to get at is if you moved the Ex5.5 box behind the ISA server, then it stopped working. If this is the case...
Do you have the smtp and pop3 packet filters enabled? (Access Policy\IP Packet Filters)
Have you published the mail server using the server publishing wizard?
Other than those things being messed up, I am not seeing anything in your network configuration that jumps out as wrong.
If all of the above is done and correct, I am stumped.
Let me know if this helps,
I re-read Alun's post again, are you trying to telnet from the internal network, or an external account? I have a dial-up machine that is not on my main network that I use through my personal ISP to hammer on my ISA.
I can't telnet through the ISA to my external IP either. Can you telnet into your mail server using the internal IP?
Not that this fixes the inbound mail problem, but it would help us to know if your mail server was published correctly.
[This message has been edited by Dominicon (edited 01 October 2001).]