Thank you for your input. We will consider this functionality in upcoming releases, if it will be popular.
Everyone, please continue voting for this feature if you consider it important.
- SUSérgio Basto commented
from https://docs.plesk.com/en-US/onyx/administrator-guide/mail/antispam-tools/dkim-protection.59433/ "Important: DKIM validation works only for real domains that use the Plesk's DNS server. It will not work if you use an external DNS service because Plesk must be able to add the DKIM-related records to the DNS zone of a domain. "
But I could enable local dns (still not use it) , go to to "Use DKIM spam protection system to sign outgoing email messages " in mail settings uncheck use dkimi, apply, check dkim again , apply bach to dns settings and you have there default._domainkey.yoursite.com. TXT v=DKIM1;
just copy that to real DNS !