[exim] Unknown Sender

Hi,

I was wondering why mail delivered to a Hotmail.com account from my Exim
4.54 MTA arrives as "Unknown Sender" when sent from MS Outlook 2000 and gets
put in the "Junk Mail" folder, but when sent from Thunderbird 1.0.7 on a
Fedora Core 4 machine sender is known and it gets put in the Inbox.

The Thunderbird client is setup to use IMAP-SSL, and Outlook 2000 is setup
to use POP-SSL. Both are courier servers and configured essentially the
same.

Can it have something to do with this?:

auth_advertise_hosts = ${if eq{$tls_cipher}{}{}{*}}

Thanks for your help!
Robert