In my Mail Server Survey for March 2004 I scanned over 106,000 mail servers. The results can be found here:
The main conclusion of this survey is this: "[...] Sendmail, Postfix and Exim make up for over 70% of the mail servers in this survey. As these servers are usually running under a *nix-like operating system (Unix, Linux, FreeBSD, Solaris, etc.) it shows the power of *nix-like operating systems in the mail server market.""