documentation?

Another issue:
K-9 Thunderbird email client for Android is not able to parse//display correctly the emails with ARC from Plesk.
Other email clients like TheBat struggles to display the email.

The only difference I can see :
On emails which the ARC is added by Plesk, ARC headers are on top of the email

Outloook.com for example is adding ARC headers after Received-SPF

Where is documentation about what was implemented with respect to ARC?

Without ARC, plesk can no longer be used as a mail server. Once Google rejects emails from servers without Arc support, other providers will quickly follow.

According to the Google support page linked bellow, having ARC becomes a requirement from February 1, 2024 for forwarding email to Gmail.

https://support.google.com/mail/answer/81126?hl=en&sjid=1378860706429114463-EU#zippy=%2Crequirements-for-all-senders

Oh yes, Plesk definitely needs ARC. Without ARC, e-mail forwarding is rendered useless if any sender (to a forwarded address) has implemented a DMARC policy other than none.

This is not a matter of Plesk admins requiring this feature, this is a must-have if Plesk wants to continue to be used as a mail server. The internet is moving on, all major e-mail providers push the implementation of DMARC and support ARC.

Since we cannot control sender domains we have no option to mitigate this issue on the Plesk side if e-mail forwards are in place.

Please implement this!

+1

This has become an issue for me as well, to the point customers are now looking at alternate solutions.

ARC is already implemented / supported at major mail providers: http://arc-spec.org/?page_id=79 (e.g. Gmail, Office 365) and Mailman.

Postfix compatible implementations exist via (at least) OpenARC (Trusted Domain Project) and Rspamd

DMARC will fail similar to SPF on forwarding, which is e.g. supported and widely used in Plesk. You implemented SRS to prevent SPF to fail, ARC is the same for DMARC.