Gmail does not tell you when a message you sent has been read. An email tracking extension adds a small tracking pixel to outgoing emails so it can notify you when the recipient opens the message, which is useful for sales follow-ups or confirming an important email was seen.
How email open tracking works
Most email tracking extensions insert an invisible image into the email you send. When the recipient's email client loads that image, the extension records it as an open and notifies you, often with a timestamp. This is why tracking is not always accurate: some email clients block images by default.
Tracking link clicks in an email
Beyond open tracking, some extensions can also track whether a recipient clicked a specific link inside the email, by routing the link through a tracking redirect first. This is more reliable than open tracking since it does not depend on images loading, but adds an extra redirect step for the recipient.
Privacy considerations for email tracking
Tracking pixels raise privacy questions because the recipient is usually unaware their open is being recorded. Some regions have rules about tracking without disclosure, so consider whether tracking is appropriate for the relationship, and be aware some email clients now block tracking pixels by default.