A mutation the reviewer ran (if ($box !== null) -> if (true)) left every
test green: CloudReady hand-rolled the same null-mailbox guard as
SendsFromMailbox, but nothing exercised its null branch. Added the
missing test, then extracted the decision itself (null mailbox -> no
sender fields, no_reply -> no Reply-To) into one shared method so an
Envelope-shaped and a MailMessage-shaped consumer can no longer disagree.
nexxo