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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.
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.
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.)