Exchange 20003 – can’t sent email externally

I have setup Exchange 2003.
My Domain name is regiter with DYNDNS (HOMENY.selfip.com).
I installed DYNDNS client on my pc.
On my LAN i am able to sent and receive email within my own network. From outside, example – i sent a email from hotmail, yahoo or gmail to user1@homeny.selfip.com, i am able to receive it. i have no problem getting emails. But the only thing is that i can’t send externally.
I am new at this.
Please help !

Thanks