Handle bounce notifications from email providers

[stephank]

Some email providers can notify us when an email has bounced or otherwise been rejected. For example, AWS SES does this. We could act on these notifications to temporarily block addresses or even domains.

I feel like this may be best done in a separate process, with Redis as the communication channel. Then the broker just has to do a simple Redis call to check for these temporary entries. (Because we otherwise only check config / files.)

(Idea from #96)

[stephank]

Additional goal: provide user feedback. I'm leaning towards a generic 'mail delivery failed' type of notice to the user, because I'm not sure if there are security implications to displaying the SMTP error to the (at the point untrusted) user.

Displaying feedback in the email loop would also require some notification mechanism between the browser and broker, so we can actually display the delivery error when it happens. Could just be a polling type of thing.