Thank you for your quick reply. Please bear with me as I may have some dumb questions!
You said that:
"Please make sure that you have added the domain only as email domain in ispconfig and not as transport. "
Where can I add a transport domain? I even don't know how and I cannot find adding a transport domain in ispconfig.
Regarding the postfix main.cf file, I haven't touched it. My server installation is a clean out of the box installation.
I should add one note here that, I have asked our network provider to add the DNS entries including the MX ones in their DNS server. So, I guess I am not using mydns on local sever. Could it be related to my problem?