This installation of Exim includes Exiscan content scanning support,
from http://duncanthrax.net/exiscan-acl/, which is documented in
${PREFIX}/share/doc/exim/exiscan-acl-spec.txt.  The ports tree
includes various content scanners that might be useful, e.g.
mail/p5-Mail-SpamAssassin and security/vscan.

When Exiscan is actually enabled in the Exim configure file, great
care should be taken to test upgrades, since Exiscan updates tend
to not maintain backward compatibility.