As Rainer suggests, a single DKIM key for all and a CNAME record for each domain is arguably the best solution.xaf (first post! French sysadmin since 1999)
In case one has to manage many domains and subdomains, it is a good solution for managing the zone files, but here it was another issue. DKIM in combination with DMARC is a way to authorize someone else to use ones domain, since he was ordered to do so. Anybody can sign mydomain.de with DKIM, but unless DKIM doesn't find a matching Public key under SELECTOR._domainkey.mydomain.de, it is invalid. The only one, who can place the corresponding public key, is the person who has access to my DNS, that's me. I can copy this public key created from my mailing service provider, so he is authorized to use mydomain.de. A more comfortable way is, he puts the public key somewhere on his own server and I create a single line CNAME:
FOREIGN-SELECTOR._domainkey.mydomain.de CNAME customer1234.mailing-service-provider.de. Best wishes Rainer
smime.p7s
Description: Kryptografische S/MIME-Signatur
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