To use change "Event Owner" to select the user delegating their calendar ensure the delegator is also an Inbox user that has authenticated with Inbox in Outlook Integration or Einstein Activity Capture.
If using the Email Composer in Lightning Experience ensure the user's Salesforce Email address has been authenticated with their O365 account. Users should be prompted for this authentication the first time they send an email within Salesforce after receiving the Inbox permission. This connection can also be set from Settings | Email | Connected Accounts.000384661

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