How to configure DKIM policy in Plesk?
After enabling DKIM for a domain, two DNS records are added at Domains > example.com > DNS Settings. A DNS TXT record "_domainkey.<example.com>" contains a DKIM policy which can be adjusted.
For example, the default policy "0=-" means that all emails from this domain will be signed with a DKIM signature. In order to configure DKIM policy, edit this DNS TXT record.
For more information about configuring DKIM settings, refer to DKIM.org.