Hello Michael,
Regarding the check that you wish to maintain only some users for
synchronisation will be via
1) Maintain the standard communication email address for those BP who are need for synchronisation.
2) In the Exchange Side maintain the permissions only for these BP's
Regarding synchronisation of External attendee.
1) You will have to maintain the External attendee as BP in the CRM system where they have there email address maintained .
2) These external users cannot be the organiser of the meeting. They only can be the attendee.
Please note that invitations sent by an external ID to a CRM user are not transferred to CRM system as the organizer is not an CRM user. When an appointment is created with external and crm users as attendeesthe external email ids are also neglected .This is the normal expected behaviour according to SAP standard design.
Thanks
Willie