IIRC these run some sort of OEM Clavister Core ...
What IP adresses do you use on those interfaces?
You mean you send a email from the internet, that arrives on a machine in your LAN? Or what?
What IP is used for the web-server? What DNS server(s) are configured on that machine? What is the destination of the mail (domain, and behind which physical interface of the DLINK box is the target mailserver to be found)? Do you use NAT and/or SAT for the connection?
What do you see in the logfile of the web-server about why the maill is not sent?
Well, I've configured quite a lot of Clavister boxes, I prefer the original, not the OEM clone. Your problem is probably not DLINK specific but probably some sort of NAT/SAT and/or DNS problem.