On the Organization FQDN page, enter the externally accessible FQDN for your Internet-facing Exchange 2016 Mailbox server. ...

On the Organization FQDN page, enter the externally accessible FQDN for your Internet-facing Exchange 2016 Mailbox server. %BRAND_OFFICE_365_SHORT% uses this FQDN to configure the service connectors for secure mail transport between your %BRAND_EXCHANGE_ONLINE_SHORT% organizations. For example, enter "mail.contoso.com". Select next.