A hybridExchange environment can provide the smoothest migration to Office 365, or it can be used to keep a mix of onpremisesmail users and Office365mail users for an extended period of time.
A hybrid deployment provides a unified email experience for the organization, enabling users with mailboxes in the onpremisesExchangeServerenvironment and users with ExchangeOnlinemailboxes to find each other in the global address list, and to send, receive, and reply to email regardless of which system is hosting their mailbox.