Why do I get 550 5.1.1 User unknown message ???

[countrydj]

Hi...
I am using Centos 5 on my linux server.

This is my problem:
Using my own server's SMTP (Relaying is allowed from my ip number),if I send an email, using Eudora or any other mail client, to one of my customers it will not send. It gives a 550 5.1.1 User unknown message.
I can send email to any of my other clients without problem.

Using my ISP smtp server, the same email will send and is received by the client.

There is obviously something missing on my server that is refusing to accept this particular email address.

Can anybody suggest anything that may cause this to happen???

Thanks,

John C

John C
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countrydj UK