Microsoft choose Postfix

merlin @ stalker ~ $ host -t mx microsoft.com
microsoft.com mail is handled by 10 mail.global.frontbridge.com.
merlin @ stalker ~ $ telnet mail.global.frontbridge.com 25
Trying 216.32.180.22 ...
Connected to mail.global.frontbridge.com.
Escape character is '^]'.
220 mail30-va3.bigfish.com ESMTP Postfix EGGS and Butter
^]
telnet> quit
Connection closed.

And you say - Exchange ... :)

frontbridge.com is a domain owned by Microsoft. Judging by the name - something like a "bridge", "first bastion".
')
Of course, there is nothing difficult in setting up relay to postfix, and the real (but shadow) server will be anything. The question is different - on the front of email, Microsoft doesn’t have Exchange at all, but Postfix, and not Windows at all (since Postfix is ​​not ported to it).

btw thx to Kessian