In a hybrid deployment where some mailboxes are on-premises and others are in Exchange Online, and the MX records are pointed to Exchange Online, Exchange Online Protection (EOP) provides anti-spam and anti-malware filtering for the on-premises mailboxes. Therefore, to ensure these on-premises users are protected, an Exchange Online Protection (EOP) license must be purchased for each on-premises mailbox. This solution minimizes costs as it only requires purchasing the necessary EOP licenses for the remaining 3,000 on-premises mailboxes.
To ensure that Exchange Online services can access the necessary on-premises virtual directories in a hybrid deployment, the two critical virtual directories that should be accessible are EWS (Exchange Web Services) and Autodiscover. EWS enables various functionalities between Exchange Online and on-premises Exchange, such as mailbox access and availability information. Autodiscover is essential for configuration and connectivity settings. Making these two directories accessible ensures seamless integration and communication between the on-premises Exchange environment and Exchange Online.







