Per-install email configuration


I'm looking for some way of changing email configuration per install
(eg. different mail server).

It should work a bit like the config/database.yml which I keep per
install (eg. it isn't part of the repository) and then just be
included in the config/environment.rb

Is this a sensible way to go about it and if it is, how can should I
implement it?

Is there some other, more clever way of doing it?