Help Please!

I have managed to succesfully (or not as the case might be) setup ActiveSync on exchanger server 2003 through the IIS manager. Although the active sync is working i now have a problem as follows;

1) i can receive e-mail from external sources.
2) I can send and receive mail from internal address's.

I no longer can send out mail to external address, i think this is a problem with name resolution?

PS prior to setting up the activesync all was working well.

Please can anyone help

Many Thanks.