No need to manage this seperately, it's all related and only brings in a single dep.

This helps servers hosting many actors, and it helps us when delivering to lots of people on the same server; we only need to make 1 request per server.

MailMate, for example, using quoted-printable encoding when lines are about 80 chars long. Blegh!

RFC822 makes Spamassassin unhappy and I learned isn't actually compliant with revisions to the message spec!

When we receive activity that has a collection, we see if any of the other actor recipients have the collection set as their followers. Then we can encode it with plus addressing. If we can't do that we drop it. Not ideal, but it covers most use cases. Next up we can encode arbitrary collections like "some/path/to/thing@example.com".