In the chain sender => list.archlinux.org => list members, I suspect it's Mailman (on the Arch list server) that does not support 8BITMIME; since OpenDKIM on that server succesfully verified the DKIM signature, the mail arrived there as 8bit, but is being distributed to list members as 7-bit.
Could the Arch listserver set "disable_mime_output_conversion=yes" in its master.cf at the point it is handing over mail to Mailman? (not globally!) As suggested in https://www.postfix.org/MILTER_README.html#workarounds
This way, 8-bit messages will not be converted to 7-bit QP or base64 when going through Mailman, and arrive intact at 8BITMIME capable recipients.
(After Mailman, Postfix' smtp client will still convert messages to 7-bit when delivering to non-8BITMIME capable recipients, which will still break DKIM validation for them, but non-8BITMIME capable DKIM-validators will have issues with a *lot* of mail anyway, forwarded or not.)
On Mon, Oct 31, 2022 at 00:04:29 +0100, Jaron Kent-Dobias wrote:
On Sunday, 30 October 2022 at 23:57 (+0100), Jaron Kent-Dobias wrote:
Confirmation: when Arch Linux forwards a base8 encoded email to the list, it mangles the DKIM. It does appear to be an Arch problem!
One last email: what the lists are specifically doing is rewriting 8bit encoded emails in a base64 encoding.
From the email in my sent folder:
From the email I received from the list:
Rewriting the body in a new encoding breaks DKIM.