DEBUG SMTP: Sending failed because of invalid destination addresses
This is the message which tells you about the problem. (Note that authenticating yourself to the server won't make an invalid destination address suddenly be valid.) If you still don't understand the problem, though, your best bet is to ask the administrator of your SMTP server to help you out.
Joined: Aug 09, 2002
The target email address is definitely valid and I've been using it for the past 10 years.
As mentioned, Outlook Express managed to send it out.