Depending on when you see this message it means either the email address you
entered during account setup is not valid, or the email address you entered
in the To: field (or the address of the Contact you selected in the To:
field) is not valid.
A valid email address won't have spaces, doesn't start or end with a period
or other punctuation, and is very limited by what special characters can be
used in them.
What version of Outlook are you running? This should be included with any
requests for support. Also, please provide what kind of email account you
have set up (POP3, IMAP, Hotmail, etc).
Joe