"Domain of sender address duff@duff.com does not resolve"

I'm trying to send an email from a mail client (outlook express) running on NT 2000 via a linux box running sendmail. Everytime i try i get the ff message from sendmail: The message could not be sent because the server rejected the sender's e-mail address. The sender's e-mail address was ' mailto:'daProwler@blueyonder.co.uk' duff@duff.com' mailto:'daProwler@blueyonder.co.uk' . Subject 'Anime - Thundersub', Account: '192.168.50.1', Server: '192.168.50.1', Protocol: SMTP, Server Response: '451 4.1.8 ... Domain of sender address duff@duff.com does not resolve', Port: 25, Secure(SSL): No, Server Error: 451, Error Number: 0x800CCC78 I can 'receive' email but cannot send. I seem to recall reading something about setting the dns of the NT machine but i;m not sure what or why. I've checked the sendmail site but that's cryptic as it usually is. Anyway i believe the problem is on the NT side of things. Clifford