When I try to send email to anyone it says:

The message could not be sent because one of the recipients was rejected by
the server. The rejected e-mail address was '.
Subject 'hey', Account: 'mail.clearwire.net', Server: 'mail.clearwire.net',
Protocol: SMTP, Server Response: '550 5.7.1 ...
Relaying denied: You must check for new mail before sending mail.', Port: 25,
Secure(SSL): No, Server Error: 550, Error Number: 0x800CCC79

This is just one example, it happens to everyone I try to send mail to.