E-mail problems for users

Hi you all, if anybody out there could be able to help me I would be grateful !
Here is my problem my company has recently changed from a satellite system to a wireless system (Internet).
I had to remove the satellite system and configure the wireless on our server (SBS 2003) so that the user can get internet access (all using XP Pro and on domain). I confined the wireless router via RRAS using PPPOE. The internet is working fine the only problem is that the e-mail is working properly we are able to receive e-mail but not send any mail.
We did not configure the server to use Exchange
So pls help I am stumped on this 1