Getting qmail working with DI-624

After too many problems to discuss with Linksys routers, I bought a
DI-624 wireless router with 4-port switch. It has all of the features
I could wish for including IP chains and virtual server. It's plenty
fast for me and the wireless connections don't drop out like they did
on Linksys.

I have one remaining problem. Qmail no longer works properly through
the router. It works just fine through the switch (local IP). I can
reach the ports (25 and 110) from outside (using telnet to test port
accessibility). For some reason, qmail doesn't respond when the access
is from the Internet.

My qmail runs on a separate Linux server in my LAN network.

Does anyone have any ideas?