Legitimate email forward to external address

[DavidePrincipi]

The Forward messages option under the Mailboxes page does not work for external recipients.
SMTP servers often reject or discard forwarded messages because NS8 is not listed as an authorized sender in the original domain’s SPF record.
Example: a message from user1@external.example.com to user2@ns8.example.org cannot be forwarded to user2@external.example.com since NS8 is not in the SPF record of external.example.com.

Proposed solution

• Add an option in the cluster-admin UI to rewrite the SMTP envelope FROM and MIME FROM headers, using the NS8 recipient address as the new sender.
• Enable the Dovecot editheader Sieve module to let users adjust their filters to send messages with a legitimate sender address.

Additional context

See also

Discussion https://mattermost.nethesis.it/nethesis/pl/6wwuwbbmztynuebpqgmjoi8skh

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Thanks to @lucagasparini