From: DeskNow Knowledge Base
http://www.desknow.com/kb/

javax.mail.AuthenticationFailedException
http://www.desknow.com/kb/idx/11/019/article/

If every message that you send bounces back with this error description ( javax.mail.AuthenticationFailedException ):

probably your DeskNnow server is configured to use SMTP Forwarding to send all outgoing mails through another SMTP server (ex. your ISP's). This error can occur if the username/password that you specify in Administration/Mail/SMTP Out is not accepted by the remote SMTP server.