[FIX] mail: fix bounce email addressing

- The "From" should not be the administrator user but
  the postmaster/system, and should be an address that
  ignores double-bounces. Using the default bounce
  email accomplishes both.

- The "To" recipient should be the "envelope from"
  sender when available, usually as provided by
  the "Return-Path" header set by the upstream MTA.
  If not present, fall back to the "From" of
  the original email.

- adapt tests to verify the behavior wrt Return-Path

0 files changed, 0 insertions, 0 deletions