Reverse DNS?

I have received a message from somebody saying the following:

Your first MX record is invalid:

http://www.samspade.org/t/lookat?a=greencell.co.uk

(remove spaces from the url)

mail.greencell.co.uk [213.208.89.218 ] is not functioning.

Our mail server is working on this IP address. I followed the link to samspade but can’t see what the problem is. Any ideas? It suggests that there is no reverse DNS, do I need it and how do I set it up if I do?