Ref T13222. If we receive a message and nothing processes it, we normally try to send the user an error message.
Just skip the "send them an error message" part if any recipient was reserved, so if you "Reply All" to a message that is "From: noreply@phabricator" you don't get a relatively unhelpful error.
This also makes sure that the "void" address doesn't generate bounces if the "From" is a valid user email address (e.g., with `metamta.can-send-as-user`). That is:
- Phabricator needs to send a mail with only "CC" users.
- Phabricator puts the "void" address in "To" as a placeholder.
We don't want that mail to bounce to anywhere. Normally, it won't:
- From is usually "noreply@phabricator", which isn't a user, so we won't send anything.
- The message will have "X-Phabricator-Sent-This-Message: true" so we won't process it at all.
...but this is another layer of certainty.