Outbound bcc

[NetserveMalta]

Hi everyone, I'm trying to implement automatic outbound BCC using a trusted System Sieve script bound to the DATA stage of the SMTP inbound listener.

The script works correctly and the redirect is generated, but messages to remote recipients are rejected because the envelope From is empty (<>), causing DMARC failures on the receiving side. Redirects to local recipients are delivered though, Stalwart apparently is not checking the DMARC policy when it detects a local message.

I tried using set "envelope.from" "sender@example.com"; before the redirect, but as soon as that line is present, the redirect is not generated at all.

I also tried rewriting the From header with deleteheader + addheader before the redirect, but the envelope From remains empty regardless.

Here is my current script:

require ["variables", "envelope", "editheader", "copy"];

if envelope :is "from" "sender@example.com" {
    deleteheader "From";
    addheader "From" "sender@example.com";
    redirect :copy "bcc@example.com";
}

Is there a way to set the envelope From for the redirected copy without suppressing the redirect itself? Or is there another recommended approach for outbound BCC in Stalwart? The official doc is a bit obscure in this regard.

BTW, is it correct to use the System Scripts or should I use the User Scripts?

Thank you!