Once you have everything upgraded, you can go to the domain > Mail > under Preferences there is an option to enable DomainKeys. Keep in mind that you have to first enable the feature under Server > Mail.
But DomainKeys doesn't work properly unless you add a public key to your DNS record. Plesk isn't doing that so the status of your DomainKey will be marked as unknown and untrusted by most mailservers.
What we need to make this work properly is the public key (it should be the same for every domain on the server) and to attach the appropriate record to DNS.