How is email tracking achieved in SMTP?

Email tracking in SMTP is primarily achieved through the use of unique identifiers and message-id headers. Each email sent via SMTP can include a unique message identifier, which serves as a distinct reference for that particular email. This identifier allows email servers and clients to track the status of the email, indicate whether it has been delivered to the recipient's inbox, and show if the email has been opened or engaged with afterwards.

Additionally, the message-id header is crucial because it helps maintain a thread of communication, linking replies and follow-up messages back to the original email. This tracking mechanism is essential for both the sender and email service providers to analyze engagement metrics, such as delivery rates and open rates.

While tracking pixels can allow for tracking of whether an email has been opened, they are generally utilized alongside unique identifiers for comprehensive tracking. Thus, while other methods like session logs for defect monitoring and IP address references can provide insights into email interactions, they do not facilitate the same level of detailed tracking as the unique identifiers and message-id headers in the context of SMTP.