Switch from resent headers to rewriting From for DMARC compliance

[daniel-lerch]

Instead of sending original messages with resent headers, we reconstruct only a few headers (Date, Subject, To,Cc, ReplyTo), namely not From which is rewritten to comply even with the strictest DMARC policies.

• Resolves Unify distribution list types in a single DMARC compliant type #91

[Copilot]

Pull request overview

This PR updates Mailist’s email relay pipeline to forward messages in a more DMARC-compliant way by reconstructing outgoing messages (notably rewriting From to the configured sender and using Reply-To for the original sender), replacing the prior Resent-header-based approach.

Changes:

• Replace Resent-header forwarding with a single PrepareForward() flow that rebuilds selected headers and rewrites From for DMARC compliance.
• Update relay job logic to apply recipient override by mutating To/Cc on the prepared forwarded message.
• Update SMTP delivery to send with an explicit envelope sender/recipient (decoupling SMTP RCPT TO from message To/Cc headers).

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 4 comments.

File	Description
server/Mailist/EmailRelay/MimeMessageCreationService.cs	Removes Resent-based preparation and introduces PrepareForward() that rebuilds headers and rewrites From/Reply-To.
server/Mailist/EmailRelay/EmailRelayJobController.cs	Switches to the new forward preparation and applies OverrideRecipient by clearing/modifying To/Cc.
server/Mailist/EmailDelivery/EmailDeliveryJobController.cs	Sends via SMTP using explicit envelope sender/recipient to avoid relying on message header recipients.
server/Mailist.Tests/MimeMessageCreationServiceTests.cs	Adjusts the test to provide serialized headers and calls the new PrepareForward() API.

Comments suppressed due to low confidence (1)

server/Mailist.Tests/MimeMessageCreationServiceTests.cs:66

• This test still only asserts the resolved display name. Since PrepareForward() now rewrites header fields for DMARC compliance, it would be useful to also assert key behaviors (e.g., From address is the configured sender address, Reply-To is preserved/falls back as intended, and To/Cc are copied from the original headers).

        InboxEmail inboxEmail = new(
            uniqueId: 1,
            subject: "Test",
            from: "support@example.com", // Armin Adendorf in demo.church.tools
            sender: null,
            replyTo: null,
            to: "test@example.org",
            receiver: "test@example.org",
            header: header,
            body: body);

        var mimeMessage = await emailRelay.PrepareForward(inboxEmail, TestContext.Current.CancellationToken);
        if (mimeMessage.From[0] is MailboxAddress from)
        {
            Assert.Equal("Armin Adendorf", from.Name);
        }

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