deliverability

This skill answers one question: does the mail arrive where it can be seen, and what do you do when it stops.

Not what the message says, and not who receives it: those belong to the neighbors. What lives here is the construction of the sending side, how it identifies itself to the receiving side, what it uses to prove the identity is its own, the reputation it has accumulated, and what to do when a provider stops letting it through. Three properties make this unlike anything else in the library.

1. The decision belongs to a third party. The mailbox provider decides, by its own rules, and it publishes those rules only in part.
2. The most important event is invisible. An unsubscribe is an event and a complaint is an event. None of the providers named in this skill report which folder they chose: the provider accepted the message and said nothing about where it put it.
3. Reputation belongs to a pair, not to a sender. The same domain can stand well with one provider and badly with another, and that is an ordinary state rather than a fault.