One of the things that makes Exim a powerful mailer construction
kit is that it has a robust set of string expansions,
which can be used to implement conditional logic among other things
(examples include 1, 2,
3, and 4).
However, this power comes with an obscure, compact syntax that's
more or less like a Lisp, but
not as nice, and in practice is surprisingly easy to get lost in.
String expansions have an 'if' so you can implement...