Exim and hotmail

Anyone know what criteria hotmail is using to determine what is "bulk mail"?

I have an exim 4.34 mailserver on linux, sending messages (not relaying)
in plain text format to hotmail and it goes to the "bulk mail" folder
unless I'm replying to a message that was sent to me by hotmail.
Some background info - using static ip, not dialup, no rbl entry,
sending to user in "to" field, no mass mailing, valid domain name on server.

Any ideas?