Why standard Salesforce notification emails are not sent out when a record is assigned by Q-assign
it is not possible to trigger the standard "Send Notification Email" alert that's available when changing record ownership via the User Interface when updating record ownership via the API or with any other automated ownership change process.
If you're updating a manageable number of records an administrator may choose to create a workflow rule and email alert action to automatically send notification emails upon updating record owners via the API. Salesforce Knowledge Article: https://help.salesforce.com/articleView?id=000233492&type=1
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