Exim's biggest advantage over
sendmail is its
human readable config file format, which it inherited from
smail.
Current versions of exim are much more powerful than
smail ever was. Fairly simple changes to the config
file can do some amazing things without a lot of
pain.
(Mostly, I am concerned with easy maintaince of configuration information such as
mailing lists, filters, virtual user and domain addresses,
spam blocking, etc., rather
than elaborate routing and mangling of headers.)
The penalty, of course, is a huge set of documentation
(which smail also lacked IMHO). But this is pretty much
balanced out by config files that are still human readable.
(And I've yet to read it all, as it wasn't necessary to
get what I needed done.)
I'm sure that sendmail is still more flexible than exim.
But then, assembly language is still much more flexible than C, not that this is a very good comparison. (But is sendmail's macro based configuration system as easy to use and as
powerful as exim? idunno, I haven't tried it. I found exim first. You try both and tell us all. 8-P)