Message sent with mutt rejected by server

[Anthony Ivanoff <a-i@xxxxx>]

Hello,

I've sent a message using mutt, one minute later I got it back. There was the 
following notice:
- - - - - - - - - - - - - -
... while talking to mx1.yandex.ru.:
>>> DATA
<<< 550 5.7.1 Policy analysis reported: Yandex users - please turn on SMTP
+auth, others - please use SMTP server of your provider
+rcpt=<xxx@xxxxxxxxx>
550 5.1.1 <xxx@xxxxxxxxx>... User unknown
<<< 503 5.5.2 Waiting for RCPT command
- - - - - - - - - - - - - - 

Any suggestions on how to send my letter anyway?