been opened. Return-receipts Another kind of receipt can be requested, which is called a DSN (delivery status notification), which is a request to the recipient's email server to send the sender a notification about the delivery of an email that the sender has just sent. The notification takes the form of an email, and will indicate whether the delivery succeeded, failed, or got delayed, and it will warn the sender if any email server involved was unable to give the sender a receipt. DSNs are requested at the time of sending by the sending application or server software (not inside the email or headers itself), and the sender can request to "Never" get any, or to "Always" get one, or (which most software does by default) only to get DSN if delivery fails (i.e.: not for success, delay, or relay DSNs). These failure DSNs are normally referred to as a "Bounce". Additionally, the sender can specify in their DSN request whether the sender wants their receipt to contain a full copy of their original email, or just a summary of what happened. In the SMTP protocol, DSNs are requested at the end of the RCPT TO: command (e.g.: RCPT TO:<> NOTIFY=SUCCESS, DELAY) and the MAIL FROM: command (e.g.: MAIL FROM:<> RET=HDRS). Email marketing and tracking Some email marketing tools include tracking as a feature. Such email tracking is usually accomplished using standard web tracking devices known as cookies and web beacons. When an email message is sent, if it is a graphical HTML message (not a plain text message) the email marketing system may embed a tiny, invisible tracking image (a single-pixel gif, sometimes called a web beacon) within the content of the message. When the recipient opens the message, the tracking image is referenced. When they click a link or open an attachment, another tracking code is activated. In each case a separate tracking event is recorded by the system. These response events accumulate over time in a database, enabling the email marketing software to report metrics such as open-rate and click-through rate. Email marketing users can view reports on both aggregate response statistics and individual response over time. Such email tracking services are used by many companies, but are also available for individuals as subscription services, either web-based or integrated into email clients such as Microsoft Outlook or Gmail Email tracking services may also offer collations of tracked