sender: User@b.domain
    from: User@b.domain
      to: User@b.domain
      cc: User@b.domain
     bcc: User@b.domain
reply-to: User@b.domain
env-from: User@b.domain
  env-to: User@b.domain
  sender: User@d.domain
    from: User@d.domain
      to: User@d.domain
      cc: User@d.domain
     bcc: User@d.domain
reply-to: User@d.domain
env-from: User@d.domain
  env-to: User@d.domain