Smail's biggest advantage over sendmail
and other MTA
's is its human readable config files. Smail tried to integrate everything useful
into its functionality, so that you didn't need to compose
the perfect config file to get it to do what you want
. You could use a very simple config file, or just use
compiled in stuff and it would work fine.
Thus, it wasn't necessary for it to be as flexible as sendmail.
However, smail is, as far as I can determine, totally obsolete
and unmaintained. (I used it for many years and liked it, and still use it at home, at least until I upgrade.) There are critical patches for it that have been
floating around for it for more than two years and have not
been integrated at all. smail is vulnerable to several relaying attacks, and thus is not suitable
for use on internet.
Exim appears to have a complete superset of smail's
functionality (except uucp routing), a similar config file format, and even shares
some of the same authors.
Since even in the uucp community, nobody uses bang paths anymore (they use internet format addresses),
I think it is safe to say that Exim has totally superseded smail.
Will wonders never cease! It appears that on July 20, 2001,
a new release of smail came out that probably fixes the security
problems in smail. Their release announcement also acknowleges
exim, along with several other MTA's, and actually sort of recommends them over smail..