Inbound Message Delivery - Administrators can specify the domain location for incoming email delivery. This allows you to specify whether the domain is hosted locally or partially/entirely on an external server. The following options are available:

Local - Select this option if the mail server is hosted locally.
External (use MX record) - Select this option if the mail server is hosted partially or entirely externally. Messages will be delivered based on an MX lookup. Select the option "Deliver locally if user exists" to perform a local delivery instead of external if the user exists locally.
External (use host address) - Select this option if the mail server is hosted partially or entirely externally. Messages will be delivered to the specified host address. The host address can either be entered as an IP address or the Fully Qualified Domain Name (FQDN), such as mail.yourdomain.com. Select the option "Deliver locally if user exists" to perform a local delivery instead of external if the user exists locally.