Why is sendmail is part of the system and not a package?

[Rich Kulawiec]

Having used sendmail since (quite nearly) the day it was released,
and having also spent considerable time with postfix, exim, etc.
in a variety of environments both small and quite large, I think I'm
in a position to address this.

Sendmail remains one of the best choices for an MTA.  It's quite
easy to configure for nearly all installations -- I would say that
over the many I've done, most of those required only a few lines
of changes to one of the m4 files to produce a fully-working
configuration.  It has an excellent feature set.  It's maintained
by some of the most experienced MTA people on this planet and while
I don't agree with all of their design or implementation choices,
I've learned to respect their judgment.  It's readily configurable
and customizable for some quite demanding and/or esoteric environments.
It's documented exhaustively and considerable expertise abounds.
It integrates well with just about everything, from webmail frontends
to POP/IMAP servers to mailing list management software like Mailman.

I see no reason at this time to change to another (default) MTA.

Which is not to say that everyone should run the default MTA: some
installations may require features which sendmail doesn't offer and
can't be handled by milters.  But in those cases -- where another
MTA is required -- I expect the implementor to have the expertise to
effect this change.

---Rsk