unix mail on virtual server

[imhotep]

I am having a problem sending mail from my unix web server.  We have one user set up to receive all our mail, the mail is routed to the individual users via appleshare ip when it reaches our local network.  We have two ip's assigned to us by our hosting company, a web server and a mail server.  They tell me the mail is reaching the mail ip fine.  The issue is I'm trying to send mail from telnet (eventually from a cgi script) using "virtual mail username@domain.com" and the server is not recognizing any usernames.  I can send mail to other addresses such as yahoo, earthlink, etc. just not anyone at our domain. Tech support at first told me to add users with vadduser but eventually they cut support and told me it was a LAN problem.  Any suggestions how to solve this dilemna?  Thanks