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This feature is only available if you use Gmail for work or school set up by an administrator. If you use a gmail.com account, read receipts won’t work.
To find out when an email you sent was opened, you can request a read receipt. A read receipt is sent to you as an email with the time and date of when your message was opened.
In order to request a read receipt, go on your computer, and open Gmail. Then click Compose.
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Compose your email as you normally would.
At the bottom right, click the “More options” drop down tab and select “Request read receipt” from it.
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Send your message. (You’ll see any read receipts in your Inbox. The person you sent the message to may have to approve the read receipt before you’re notified.)