EAC only performs WhoID(Contact/Lead) associations when an event is inserted from outlook

Einstein Activity Capture (EAC) syncs calendar events from Outlook to Salesforce, but WhoId association (linking Contacts or Leads as attendees) only happens the first time an event is inserted — not on later updates. This can cause confusing behavior when using Outlook Desktop for MacOS.

When you single-click on the calendar in Outlook Desktop for MacOS, a small pop-up window appears. This action creates an "empty" event in Exchange with the subject 'New Event'. Exchange notifies EAC whenever there is a change on the user's calendar, so this event immediately becomes eligible for syncing.

If the user populates the event data in the small pop-up window in Outlook and 'Send/Save' the event before EAC syncs the event, then the first time Salesforce receives the event from Exchange, all the attendee data will be available for association processing upon insertion.

However, if the event syncs to Salesforce before the rest of the event data is populated in Outlook, and the user adds attendees and hits save, then there will be no attendee data available for the association when the event is first inserted into Salesforce.

Once the 'Save/Send' button in Outlook is clicked by the user, the event receives an update sync to update the data on the newly created record, but no WhoId association processing will be performed, as EAC does not handle event updates.

So, essentially, the difference in behavior observed depends on how quickly the event is created in Exchange with all of the attendee data, how quickly Exchange notifies EAC of the event, and how quickly EAC is able to fetch the event data and sync it. This can vary over time.