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/
