550 Authentication is required for relay

[Guest]

Where I work, we use Outlook for both internal and external email. When I
attempt to reply to an external email in Outlook 2003, it is returned to me
as Undeliverable, with the notation "550 Authentication is required for
relay". I am the only one to whom this is happenning.

 

[Brian Tillman]

Where I work, we use Outlook for both internal and external email.
When I attempt to reply to an external email in Outlook 2003, it is
returned to me as Undeliverable, with the notation "550
Authentication is required for relay". I am the only one to whom
this is happenning.

Your SMTP server and ISP domains don't match. See if this helps:
http://support.microsoft.com/kb/290842/en-us