Exchange server sending mails but not receiving from outside

Hi techs!!
In my home server i’ve installed Exchange 2003. with the domain provider i’ve created a A and MX (@mydomain.com) record pointing to my public IP. Also opened SMTP and POP3 ports on my router/modem. The problem is I can send mail from my outlook to yahoo, but not getting reply from it. No delivery error message comes back to yahoo.
Now 3 questions??
Is this something to do with DNS in my server?
Or something missing in Exchnage system manager stopping mail from outside?
Or some firewall???
Please help me with this..Cheers