[Exim] Problems from Outlook XP client when sending.

I am seeing this in the logs on the mail server:

2001-11-27 12:08:18 unexpected disconnection while reading SMTP data
(after header) from (osirus) [204.244.50.130]


Osirus is behind a firewall, NAT'd, and was working, now it's not...

It seems to time out. I did a search but didn't seem to see anything that
was related (not using authenticated logins, nor is it identd timeouts)

I read fine, I can connect to the SMTP port via telnet very quickly..

Any ideas where to start, or what it could be?