Related link: http://www.falkotimme.com/projects/survey_smtp_032004.php
Falko Timme’s March 2004 survey of Internet e-mail servers reveals that Sendmail, Postfix, and Exim run over 70% of the 106,000+ mail servers surveyed. Keeping track of trends here would certainly be enlightening.
What other Internet services does FOSS do best?

qmail?
It is very surprising (to me, anyway) that this MTA has such a low penetration. I am wondering what reasons there might be. DJB's stance toward licensing his software is certainly a factor, but is it really responsible for two orders of magnitude less marketshare?