If you want to know when the email you sent has been opened, you can utilise the Gmail Read Receipt feature, which confirms that a message has been received from Gmail.
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Gmail Read Receipt _ OpenedOrNot by 500apps.pdf
1. Gmail Read Receipt | OpenedOrNot by 500apps
The Gmail Read Receipt serves as a confirmation that a message has been received by Gmail and
can be used to determine whether or not the email you sent has been opened.