Your mail server is blocking you from sending messages to email addresses outside the mail server domain. For example, if the mailserver domain is mycompany.com, you can send to
[email protected] and
[email protected] but you cannot send to
[email protected]. This relay control is an admin option on your mail server. You can change it if you hav admin access, otherwise you will have to test using addresses only in the mail server domain.
-Rob Wunderlich