If your organization is moving to a cloud-only environment, either as a cutover, IMAP, PST or stagedmigration, you will be implementing EOP as part of ExchangeOnline; in this case, you only need to plan for message handling policies in EOP. All the features in EOP with ExchangeOnline will be available.
If your organization is planning to keep their current on-premises mail setup and use Exchange Online
with Office 365, they can optionally route incoming messages through the stand-alone EOP service before
delivery to their on-premises mail system. They can also send outgoing messages through this route if
required