550 error in Outlook 2007

All

Have just developed a problem when trying to send email from Outlook 2007.
Prior to this everything was working fine.
In trying to correct this problem I have deleted my original email account
and input the details afresh but when I use the "Test Account Settings"
facility I get the same error message "The server responded: 550 Error:
Message content rejected".
Seems the suggestion is there is something wrong with the content but I am
only trying to send a test via the "Test Account Settings" facility so there
is little or no content?
Please note I can send email using Windows Mail so the problem appears to be
at my end in Outlook, (corrupted file?).
ISP is Orange and they say they don't support Outlook.
Appreciate any assistance.

Regards
Mick