I like to install zimbra on a new ubuntu server. But I can't interrupt the already running email system running on another server since last five years. Surely I like to migrate all the users to the zimbra but can't stop the existing mail server during migration phase. Hence need such an arrangement where the newly installed zimbra can start working along with the existing box. So email@example.com on both the server should be functional. Obviously at some point I need to migrate all dovecot mailboxes from the existing box to the zimbra. But during the operation, user should use both uninterruptedly. Can the split domain concept work here ? Or any better approach ?