test wrote:

How do I know they are using the correct address? Because it worked
for the first two days on Outlook 2003 .... but now its stopped
working. They pick my address form their contacts list and I've
asked them what it says and it says exactly the same as my address
for wehen they reply to my email.

If you're not receiving messages from them but you do receive messages from
others, then in the majority of cases it's the sender's problem. Without
knowing a good deal more about their setup, there's nothing you can do. Why
not invite them to post details here if they use Outlook for their mail
client.
--
Brian Tillman