Disable DKIM in exim for mailing lists

Question

I am subscribed to a number of mailing lists that don't remove my DKIM signature but mutate messages (change From) and add their own DKIM. Resulting messages have 2 DKIM signatures, one failing and one passing.

DMARC is perhaps the biggest problem for me. When I send a message via such a mailing list I started to receive dozens of DMARC reports stating that my message contains a failed DKIM signature.

How can I disable my DKIM signatures when sending messages to these mailing lists?

I have standard DKIM-related configuration for exim:

DKIM_CANON = relaxed
DKIM_SELECTOR = default
DKIM_DOMAIN = ${lc:${domain:$h_from:}}
DKIM_FILE = /etc/exim4/DKIM_DOMAIN/DKIM_SELECTOR.private.pem
DKIM_PRIVATE_KEY = ${if exists{DKIM_FILE}{DKIM_FILE}{0}}

As far as I understand I cannot check $recipients, right?

So I think I will configure my MUA (mutt) to change local part and check the local part. Something like

DKIM_PRIVATE_KEY = ${if and {!eq{$local_part}{phd+list}}{exists{DKIM_FILE}} {DKIM_FILE}{0}}

Is there a better way to do it?

Upd. I decided to go a slightly different way: add/check a custom header X-Skip-DKIM-Sig. So in my .muttrc I add the header for known mailing lists:

send-hook ~u "\
    unmy_hdr Reply-To; \
    my_hdr X-Skip-DKIM-Sig: mailing list"

and in exim I check it:

DKIM_PRIVATE_KEY = ${if and{{!def:h_x-skip-dkim-sig:}{exists{DKIM_FILE}}} {DKIM_FILE}{0}}

Answer 1

There's no need. The falling DKIM signature does not impair delivery. It's only important whether or not a valid DKIM signature is present.

For the purposes of DMARC, it is important that the domain that the DKIM signature relates to is the same as in the From MIME Header. If the mail server modifies the subject or body, it also has to modify the From header in order to be able to send something that passes DMARC. This is problematic, and many if not most mail servers fail. Explaining this to users is an ongoing problem.

ARC attempts to fix this. It's relatively new though, so adoption is patchy.