Managing accepted domains in Office365

When enabling EOP (Exchange Online Protection) the administrator is able to configure the mail flow to accept all messages from any given domain and then forward to the on-premises environment, or validate if the recipient is valid at Office365 and only the valid messages will be send through the connector to the on-premises environment. Both scenarios are valid and configured at accepted domain level in Exchange Online
Read more at source: http://www.itprocentral.com/managing-accepted-domains-in-office365/