Database Mail Help

I'm getting the following message when trying to send an the test
e-mail for Database Mail  via my Internet Provider (Verizon):

The mail could not be sent to the recipients because of the mail server
failure. (Sending Mail using Account 1 (2006-04-06T10:09:15). Exception
Message: Cannot send mails to mail server. (Mailbox unavailable. The
server response was: 5.7.1 Authentication Required).

I am configured to use Basic Authentication.  Can anyone shed some
light on the problem?

Thanks for your help,

Paul W