When implementing an Exchange Modern Hybrid connection, certain configurations can be transferred automatically from the on-premises organization to Exchange Online. The address lists will be transferred because they need to be consistent across both environments to maintain user lookup functionality. The address book policies (ABPs) will also be transferred as they dictate how address lists are presented to users, ensuring that the same policies apply in both environments. Lastly, the Messaging Records Management (MRM) settings can be transferred as they are crucial for compliance and retention policies, ensuring that data governance is maintained. Device access rules and mail flow rules typically remain local to the on-premises environment as they often contain specific configurations and logic that could be disrupted if transferred.

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.