Notes:
(Object) Email::List is a container for subscriptions. Includes a method to prune old subscriptions.
(Object) Email::List::Subscription stores info about a particular subscribe/unsubscribe. It has a match method that is called on a message and returns the a list of E::L::Subscriptions from that message.
(Namespace) The E::L::S::* parse particular types of method. So the intelligence about each type of mailing list is split across the three packages. It would possibly be better to split into list-specific and then have match_welcome, etc. functions.