Trying to preserve the original DKIM signature is unlikely to work --- Mailman tends to regenerates the headers and body from parsed copies so the details of the formatting can change, which breaks the signature. The only thing which actually works here is to strip the DKIM signature (preferably after checking it) and resigning the message when Mailman sends it. See for example https://doc.coker.com.au/internet/dkim-and-mailing-lists/.

On Fri, 10 Jul 2020 at 03:09, Sam via Cin <cin@lists.cinelerra-gg.org> wrote:

Hi Cinelerra friends,

As some of you have already noticed, modifications to Mailman were
necessary to get Gmail users back. To avoid spam, the major email
providers use methods like SPF, DKIM and DMARC.

We have adapted our mailing list so that the subject line and footer are
no longer changed, i.e. the abbreviation [Cin] and the old familiar
footer are missing. Thus DKIM is not broken and we are not classified as
spam. We have also made other changes to meet modern requirements.
Furthermore, only the mailing list address appears as the sender.

We will gain more experience with the new procedures and if necessary we
will make further optimizations. If you have any problems or other
questions, please contact me.

Sam

┌─── http://www.cowlark.com ───
│ "I have always wished for my computer to be as easy to use as my
│ telephone; my wish has come true because I can no longer figure out
│ how to use my telephone." --- Bjarne Stroustrup