Two-way reply forwarding is the mechanism that lets a reply to a forwarded email leave from the alias address instead of the real address. The mechanism operates inside the forwarding service rather than inside the mail client: the forwarded message carries an encoded reply address, the reply travels back through the service, and the service re-sends it from the alias. AliasFleet documents the mechanism in its reply and threading guides, and the same encoded-address pattern appears in the documentation of addy.io and SimpleLogin.

The encoded reply address

A forwarded message arrives in the destination inbox with its Reply-To header rewritten to an encoded address. In the AliasFleet documentation the example takes the form [email protected] , where the alias identifies the receiving identity and the encoded portion identifies the original sender.…