For what it's worth of all the open source packages on the market Exim
is definitely one of the most mature, stable, and well documented and
supported packages out there. Although there is no limit as to how much
something can be improved Exim has definitely reached the "mission
accomplished" point a long time ago in a world where a lot of software
hasn't made it to the 1.0 version. That's no small accomplishment.
