Na entrega, o endereço especificado em MAIL FROM
é colocado no cabeçalho Return-Path:
. Veja RFC 2821 seção 4.4 :
When the delivery SMTP server makes the "final delivery" of a message, it inserts a return-path line at the beginning of the mail data. This use of return-path is required; mail systems MUST support it. The return-path line preserves the information in the from the MAIL command. Here, final delivery means the message has left the SMTP environment. Normally, this would mean it had been delivered to the destination user or an associated mail drop, but in some cases it may be further processed and transmitted by another mail system.
Você não pode assinar este cabeçalho porque não o criou; o servidor SMTP de destino adiciona o cabeçalho.