email sending problem with Linux gateway

Hi,

We used to connect our local (900+) users to internet through a Netware box with two nics, and an apropriate routing configuration. This box was also running DHCP and NAMED. Our mail-web etc. servers were local, but serving both local and non-local users with the help of the portforwarding rules on the Netware gateway and an external Linux box running a DNS server for public users.

Due to some problems with Netware I decided to replace this gateway with a Linux box. I installed RH3AS and complated the necessary and identical configuration similar with the ones in the Netware box.

After the replacement, everything is fine, and even better, due to the transparent proxy (Squid) I managed to install on the Linux gateway. Except emailing.

We could receive email messages from anyone (both outise and inside) but we could not send any message to someone non-local (eg. hotmail-yahoo users).

We used to have no SMTP-POP server running on the old gateway, and our mail server was configured to send and receive messages directly. Now it can receive but can not send.

When I configure my mail server to send all messages through another one outside, that would allow relaying our messages, then everything is fine.

It can not be something related with IP resolution as it is the same Linux box which serves as a gateway to web users too.

I can't figure out what I am doing wrong.

Any help is highly appreciated. Regards.