Using Microsoft Office 2003, all the latest updates.

I get that error message now, when I click "Send" on an e-mail message to
someone who does not appear in my address book, or to whom I did not send
mail to or receive mail from before, and the mail will not go into the
Outbox, to be sent. What is causing that?

Of course I will sometimes write to people whom are not in my address book,
and with whom I have not corresponded before. Of course Outlook will not
always recognize an e-mail address I am sending a message to. Why that
should generate an error message, and refuse to send the e-mail?

This evening is the first time that has happened. Does anyone know what is
causing it, and how I can fix such strange behavior?