Hello! It’s not necessary to configure IFD for CRM just because you want to sync with some mailboxes you have in Exchange Online you would simply setup Server-Side Sync(using autodiscover or pointing directly to Exchange Online EWS) with the Exchange Online template in your CRM Org and configure appropriate account for access. You do however need to exclude the account used in the profile for accessing the mailboxes from MFA if it’s enabled. As the connection is made from CRM(Server-Side Sync which basically is the AsyncService opening it) there is no need to open the FW for inbound connection from the Internet. Of course if the CRM Server(s) in the perimeter are blocked for outgoing traffic you will need to address that. However, as you are running an older CRM version(2015 RTM) I would check that it’s still supported with Exchange Online as there might be issue’s, like TLS 1.2. If you got a Test/AT environment I would recommend you start by setup a connection from one of those. Best regards. Philip