Instead of getting all destination uids, run the search and activate with
destination_related_uid.

When MailMessage portal type is not installed, then temp mail messages will use
this class. The plan is still to remove the class, but for now just remove this
method that can overload Event.send

This rely on a change in erp5_base where Event_send does nothing and MailMessage_send sends email instead, otherwise mails will be sent for all started events.