550 Administrative Prohibition

Whenever I try to send an email through Microsoft Outlook I get a immediate reply saying Your message did not reach some or all of the intended recipients.

Subject: Subject
Sent: 20/01/2005 18:42

The following recipient(s) could not be reached:

’email address’ on 20/01/2005 18:42
550 Administrative prohibition

I have been trying to send emails for days and keep getting this email in return. I am logged on as an administrator and I can send a test email through Outlook to myself.

Thanks for your help