[exim] hostname and HELO/EHLO response

I just received this bounced message when trying to send a mail:

SMTP error from remote mailer after HELO mail:
     host mail.sourceforge.net [66.35.250.206]: 550 Don't like your 
HELO/EHLO. Hostname must contain a dot.

How do I fix this? I am using debian woody and exim 4.34