Hi
Have found this when having a quick look through google...
"This error message is coming from the mail server, not from Outlook. This
error means that the SMTP (outgoing) mail server did not recognize you as an
authorized user and rejected your e-mail message. Most ISPs (ISP stands for
Internet Service Provider, and this includes GCI) configure their mail
servers to recognize only those computer IP addresses that are on an accepted
list. If you use multiple ISPs, or just switched from another ISP, you are
likely to encounter this problem. The solution is to change setting of the
SMTP server for that mail account under your e-mail application:"
Not sure if this helps any...
Regards
Davidt