Exchange server incoming mail issue?

Hi,   I have registered a domain under godaddy.com, and I have exchange server. My question is my exchange server can send and receive emails on the same network(local network). I can also be able to send emails to outside the network( to the internet), But I am not able to receive emails from outside the network(from the internet). I have opened the port on my router and given IP address of Exchange server. My firewall didn't block port. Still I don't know why I am not receiving emails from the internet. I checked on GoDaddy DNS setting, A record, and MX record seems perfect. Can anybody help what is the reason of this ? can ISP blocks inbound SMTP connection? I can send emails outside network both via smart host and DNS MX record.