If you already have existing email users at your domain (i.e. firstname.lastname@example.org) with active email accounts prior to signing up with Starkit, then you must add their accounts in Starkit before you update your domain mail exchange (MX) records. When you update your domain MX records, all your incoming email will be directed to Starkit’s mail servers, so by creating your email users prior to changing your domain MX records, it makes sure that all incoming email will be delivered to their Starkit inboxes.
This only applies if during registration you selected to use a domain that you already own. If you registered a domain through starkitsystems.com, the domain MX records are already set up and you can add users in your Starkit account at any time.