Question : not recieving mail

I have a slackware 7.1 with kernel 2.2.16 server. There are also two clients with win98 nad win NT os-es. All three computers are in the same local network (192.168.x.0). This linux server acts as a gareway for ppp connection for clients, then it serves as a domain nameserver. Of course as I dont have a permanent connection to the internet I made a fake domian (not visible to outside world). I have also a simple firewall with ipchains ruleset. Now I want to setup mailserver so that te clients would be able to send their email through that server.
I have made 3 accounts on linux server (for 3 users) so that any user would be able to send mail from what ever computer in my local network. The problem is the mail gets queued for delivery and when on the line the mail dissapires to unknown place. I have tried to send an email to my second email address (not within my IPS), but filed to recieve it.
Is there any one with the similar configuration who succeded to make it work. If so, I would very much appreciate if one would send me the configuration file(sendmail.mc) of his/her sendmail. By the way I am using the 8.11.3 version of sendmail.

Thnx

Answer : not recieving mail

Changing 'MASQUERADE_AS(`isp.domain')' to some other domain will re-write the 'From: ' address. If you change that, you should probably remove the 'FEATURE(`masquerade_envelope')'. It's possible that doing this could cause your providers SMTP server to refuse to relay the mail. Try the change and see. If your provider's anti-sapm controls reject the mail as not being from within the ISP's domain you can always change it back and set 'Reply-To' in the client.