Ok, let’s forget for a moment the admin panel.
If the default config would be the one shown in #3804, and Strapi would check if the key is configured by querying the domainkey using fs.readFileSync('./dkim-private.pem', 'utf8') before compiling the config with dkim: false would it be impossible?
That much can save us from configuring custom email provider…
The rest would be as easy as this: Generating DKIM key with openssl - lxadm | Linux administration tips, tutorials, HOWTOs and articles