The current MTA-out using postfix has its own queue for outgoing messages.

We have no visibility into that queue, and it goes against our design to centralize most of the state in Django.

The only queue we should be using is the celery worker queue, and it should be relatively easy to reimplement SMTP delivery (grouped by domains) in a celery task, which will keep track of the delivery statuses of each MessageRecipient, which we can in turn show in the frontend as a very nice feature.

We should still support configuring a single SMTP relay for installs that don't want to directly send emails. Or to configure a socks proxy when we do, to keep stable outgoing IPs for SPF and reputation.

Also support configurable "undo send"