Hello @Captivation ,
By default, if you are trying to send an email to an invalid user, you will receive a user not found email reply immediately.
However, if you would like to get email notification for a delivered message, set the below property to true while sending mail using graph API
"isDeliveryReceiptRequested": true
If you are trying to validate an email present in your domain, you can use Get User API.
Hope this helps.
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