Replica: fix: add X-Envelope-From header

[lucaforni]

Questa PR replica la PR originale: postalserver#3512

Autore originale: @Mortza81
Branch originale: fix/add-x-envelope-from-header
Repository originale: Mortza81/postal

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Summary

This PR adds the X-Envelope-From header to incoming messages to preserve the SMTP
envelope sender (MAIL FROM) for SPF evaluation by SpamAssassin.

Background

SpamAssassin relies on message headers to determine the envelope sender for SPF checks.
If no suitable header is present (e.g. Return-Path or X-Envelope-From), SPF evaluation
may be skipped and debug messages such as cannot get Envelope-From are logged.

Motivation

Providing X-Envelope-From ensures the envelope sender is always available for SPF
evaluation, improving accuracy and consistency with SpamAssassin’s documented behavior.

References

• https://spamassassin.apache.org/full/4.0.x/doc/Mail_SpamAssassin_Conf.html

Impact

• No change to message delivery
• Improves SPF reliability for incoming mail only