Damn exim got there fast as a sendmail replacement... or maybe it didn't start in 1982 after all. [1995/6 would be a better timeline - in fact all of the sendmail replacements here started within 2 years of each other].
Personally I would ignore qmail due to licensing/maintenance issues. Postfix and exim are both good systems. I happen to have worked on exim for a long time including serious multi-million user installations, so know it better than Postfix, so I am biased by what I know.
If you want to work on personalities, both Wietse and Philip are great people - courteous, helpful and willing to give good explanations as to how and why. Unfortunately that does not hold for some of the other systems.
My experiences of sendmail are biased by sendmail 5.x
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