What is a read receipt?
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A delivery receipt confirms delivery of your email message to a recipient's mailbox, but it doesn’t confirm if they saw or read it. A read receipt confirms that your message was opened.
In Outlook, a recipient can decline to send read receipts. There are other scenarios where read receipts aren't sent, such as if the recipient's email program doesn’t support read receipts. There’s no way to force a recipient to send a read receipt.
Visit Microsoft Support to learn how to add and request read receipts and delivery notifications in Outlook.