Reverse DNS - mail error

[Gareth Bailey]

My postfix seems to be running just fine, but i recently had a problem sending mail to a particular address. The message sent to me, the postmaster, was:

host <IP Number>
    said: The server sending your mail [<IP Number>] does not have a
    reverse DNS entry. Connection Rejected. Please contact your
    Dial-Up/DSL/Network ISP Provider. Default Reject! (in reply to end of DATA
    command)

What does this mean and what do i need to do?
Any info will be much appreciated

Thanks
Gareth