Replace the MAIL FROM on the relay level

[dinosaure]

Currently, the relay keeps the same MAIL FROM as it receives. If we receives MAIL FROM <[email protected]>, we send back the email with the same MAIL FROM <[email protected]> even if our identity is not bar.com. If we can assert that the primary destination of the email is for us (like [email protected]), we should send back the email with MAIL FROM <[email protected]> to the real destination. In such situation, the real destination (let's say [email protected]) will be able to do a proper SPF check on the x25519.net domain instead of on the bar.com domain.

Spotted by @hannesm.