Server rejected the sender's e-mail address - Error 550

[arenigltd]

I am using Outlook Express Version 6

I have set up a new mail account to send mail from one of my alter-egos.

An error is returned upon sending mail.

The message could not be sent because the server rejected the sender's e-mail address.
The sender's e-mail address was 'anyone@b5xx5.com'.
Subject 'test', Account: 'B5105',
Server: 'mail.btconnect.com',
Protocol: SMTP,
Server Response: '550 <anyone@b5xx5.com>... Not permitted',
Port: 25, Secure(SSL):
No, Server Error: 550,
Error Number: 0x800CCC78

Yet I have set up other mail accounts for my other 'alter-egos' and these work fine.  And I have just copied the server details for this account.

Am I doing something wrong? 

Is there something i have to do?

Not terribly techie so please be gentle with me.

[arenigltd · Accepted solution]

I think I've found the answer to my own question....

I need to contact BT to set up my xxxx.com domain as mail relay before I can use it.  Answer was in their, actually, very useful help pages.