I would like to find out, if any body can recommend ZCS native process when registering new domain and accounts, while domain is still alive on current other (non ZCS instance) servers?

The issue:
Customer has working domain and main functionality, and current DNS MX records point to current Mail servers. To let the customer be switched over to ZCS server, we have to prepare and register domain and users within ZCS instance.

Everything until now, is ok, but...

During the period of public DNS switching and sync (while new domain and users are already registered @ ZCS), other (non-customer) users, who are at the same ZCS instance will send mails to particular customer, and result in mail delivered to ZCS newly registered accounts, not the mail server, which is defined per public DNS servers.

I'm looking for ZCS friendly way of doing this, without manual intervention in server configurations due to management overhead. Could not find advised options, or have performed incorrect keyword search. Advice is greatly appreciated.