Joining Premium Plan means having your own "Email Security Officer". The "Officer" takes care of your whole communication processes and configures the most effective system to combat exact-domain phishing attacks. Enforcing that security is very tricky and time consuming process, which requires consistent monitoring, collaboration with people and sysadmin actions. If the enforcement is done incorrectly, sent emails can be lost(e.g. your email to bank, to court, etc...), which can be harmful for business. That's why the process takes months.