support DKIM keys
When a domain is added in Email, clients can no longer manage DKIM keys in DNS (except for the single key pair entered under the Email tab).
There are numerous use cases requiring multiple DKIM keys, from simply migrating a domain from another server which has DKIM enabled (you need to add the old public key to DNS so that checking DKIM signatures on mail sent prior to the migration doesn't fail) to integrating external services which send legitimate mail from an address in your domain (eg. billing services, mail processing services, helpdesks, surveys, zoho, etc.).
Right now an admin can add multiple DKIM records (by adding TXT records), and clients have a workaround to rename the Email domain while managing their DNS records (see https://www.howtoforge.com/community/threads/client-cannot-update-dns-dkim-record.82321/#post-390423) but this should be handled more cleanly/directly in ISPConfig.