Decoupling sending and receiving emails

[dinosaure]

To get some space to really describe the failure politic when we send an email, it can be interesting to "decouple" the sending logic from the receiving logic. Currently, for any received emails, we do a Lwt.async send_the_incoming_email. But we probably should launch another fiber which waiting data for sending emails and do a Lwt.async notify_sender_fiber into our receiving loop.

By this way, we can handle properly a real policy to send emails (any emails) and when some of them fails.

A block-device which can store temporary emails can be useful too for some situations (like how we handle greylisting from recipients's servers?).