What's an MX record in DNS entry?

A staff of mine walked inside my room this morning and asked me "how to setup a mail server to receive email destined for my own domain?". This guy's got few unused PCs at home, therefore he is trying to setup a little Internet servers farm in his bedroom. He's hooked up to the Internet by Streamyx broadband service, the home user package, which does NOT allow him to have a fixed IP address, but he managed to set it up using the no-ip.com for a dynamic DNS service. This enables public to access for a sub-domain name at no-ip.com, which is then redirected to his broadband router. By the use of port forwarding on his broadband router, his webserver and mail server are publicaly accessible from the Internet. He' got so excited as he's got a little Internet server farm running in his bedroom, which he has web, mail services accessible from the Internet. Unfortunately, he failed to receive any email sent to this domain.

He's really keen on doing this, but he got stuck, frustrated as he seemed to get everything going all right, but he failed to receive email to this sub-domain of no-ip. (He sent several emails from all other email domains but not even received any single one).

Well, the crucial part here is the MX record, which he failed to setup an MX record in the DNS entry at no-ip. He does NOT understand what an MX record means. He's been googling around, what he has found is MX record is the "Mail Exchange Record", which he claimed no further information that he could find out of this !

MX Record, which represents the mail exchange record in DNS entry. This is what a mail /SMTP service needs to lookup when trying to deliver an email , which is destined for a domain For example, when someone is sending email through his/her local mail service to Yahoo.com, the local mail service will initiate a DNS lookup for the MX record of Yahoo.com. Once it's found the host of the MX of Yahoo, it will then communicate or establish an SMTP session with the MX of Yahoo, then handing over the email to the MX of Yahoo ...