Mastodon sends delete activities for every single note of an actor after
they are deleted, even though it usually sends a delete activity for the
actor itself from the start. These messages fail processing since we
can't verify the signature anymore (we have deleted the actor together
with their notes and keys with the delete activity for the actor itself,
and it's obviously not possible to look it up anymore), and stick around
in the inbox queue for all of the retries, which can take quite a while.
But since we can't have anything from actors that we can't
resolve, we can safely ignore the deletion request from them.
Changelog: Fixed
