From MSDN:
0x800CCC78 SMTP_REJECTED_SENDER Unknown sender. This is caused by
having the incorrect e-mail address in the Reply-To field.
Type 0x800CCC78 in search at Microsoft site, and you'll find a few
possible reasons. E.g., SMTP server require authorization

Signature
Kathleen Orland - MVP Outlook
Outlook Tips: http://www.outlook-tips.net/
http://www.howto-outlook.com/
> Hi
> I have an invisible email in my Outbox that is preventing me sending any
[quoted text clipped - 3 lines]
> Any suggestions welcome.
> David
Murph - 25 Mar 2008 19:07 GMT
Thanks Kathleen
The address was my alias which I have used without problem for years.
Switching it to the real address has indeed solved the problem.
Many thanks.
David
> From MSDN:
>
[quoted text clipped - 11 lines]
> > Any suggestions welcome.
> > David