pub fn plan(
envelope: &mut Envelope,
to: &MailAddress,
channel: &Channel,
) -> Result<Option<Plan>>Expand description
Who receives envelope, sent to to, when an agent’s mailbox decides it;
None when it is ordinary mail to one session. Sets the envelope’s
thread and in_reply_to where the mailbox knows them better than the
sender: a channel names its parent by its platform id.