When
Microsoft®
bought the service Hotmail was running
qmail on
unix®.
Naturally they wanted to get it up and running Microsoft
Exchange™ as soon
as possible (and some folks were quick to point out that the site continued
to run qmail).
The new owner quickly tried this plan and we presume they found that
Exchange was not up to the task - Hotmail had been stable up to that point
and the site immediately went down.
For several months the site continued to process traffic with qmail, however
with the identifying banners turned off, but a cursory look at the
details verified that the servers were still unix and the behavior was
still qmail.
Within a few months they had succeeded in moving the site to NT servers
running exchange, but it was not the cakewalk that had been expected.
My personal email archive shows Hotmail service 2 years post the MS acquistion
of the service still using qmail headers through
July 15, 2000.