SMTP Envelope
The transport-layer addressing of an email (MAIL FROM and RCPT TO), separate from and often different from the visible headers.
The SMTP envelope is the outer addressing that mail servers actually use to route a message: the `MAIL FROM` (envelope sender, used for bounces and SPF) and one or more `RCPT TO` recipients. The envelope is distinct from the `From:`, `To:`, `Cc:` headers the user sees in the message body. A message can have a `To:` header listing one address while the envelope `RCPT TO` is a hundred different recipients (a Bcc explosion), and the envelope `MAIL FROM` can differ from the `From:` header (which is exactly what DMARC alignment checks). The envelope is discarded once the message is delivered.