queue: Send delivery status notifications on failures

When we permanently failed to deliver to one or more recipients, send delivery
status notifications back to the sender.

To do this, we need to extend a couple of internal structures, to keep track
of the original destinations (so we can include them in the message, for
reference), and the hostname we're identifying ourselves as (this is arguable
but we're going with it for now, may change later).

internal/config/config.proto

@@ -2,8 +2,11 @@
 syntax = "proto3";
 
 message Config {
-	// Hostname to use when we say hello.
-	// For aesthetic purposes, but may help if our ip address resolves to it.
+	// Main/default hostname to use.
+	// This is used to say hello to clients, and by default as the domain
+	// we send delivery notifications errors from.
+	// It should be a domain we can send email from.
+	// It usually helps if our IP address resolves to it.
 	// Default: machine hostname.
 	string hostname = 1;