Figuring out whether the recipient read your message
From MozillaZine Knowledge Base
One way to tell if a person has read your message is to request a Message Disposition Notifications (MDN) return receipt (). If you select Options -> Return Receipt when composing a message it will add the "Disposition-Notification-To: ..." header to the message that requests the recipients email client to send a reply and specifies who to send the reply to. However, you can't rely upon this because many email clients and webmail let the user decide whether to ignore return receipts, or to decide on a case by case basis whether to allow the return receipt.
You can specify what Thunderbird should do when it gets a request to send a return receipt in "Tools -> Options -> Advanced -> General -> Return receipts". You also have the option of customizing the settings for an account using "Tools -> Account Settings -> Return Receipts" if you don't want it to use the global settings. The New return receipt handler extension lets you decide when you read the message whether or not to let it send the return receipt, rather than using those settings.
Another type of receipt is the Delivery Status Notification (DSN) receipt (). The only purpose of DSN receipts is to let a sender know when the recipient's server received the message. The sender cannot be sure the message will be read but DSN is less intrusive of the recipient's privacy than MDN and nearly all servers support it. Thunderbird 3 supports DSN receipts. Use "Options -> Delivery Status Notification" when composing a message to request a DSN receipt. The MiliMail project (now called the TrustedBird project) adds several features to Thunderbird such as MDN Deletion receipts and DSN Delivery receipts. Some are available as add-ons, others are built into special builds of Thunderbird. Unfortunately their last build is based on 3.1.15. See Trustedbird: Additional email security for Thunderbird.
The DSN Settings add-on provides settings to control DSN requests. The MDN Extended add-on allows the recipient of a message to send a MDN return receipt. Both support Thunderbird 3.*. Try installing the Disable Add-on Compatibility Checks add-on first, and then ignore the warning message about the add-on being incompatible when you install it.
The Notification Viewer add-on identifies and displays the status of pending and received notifications for messages sent with notification requests. You don't need to use it to get a return receipt, but it can be useful if you send a lot of messages where you request a return receipt.
If you use Exchange 2007 and the sender requests a receipt Exchange will send one receipt when you fetch/download the message and another if you click OK to tell Thunderbird to send a receipt. Nothing you can do about the first receipt.
An unscrupulous way to tell if somebody read your message is to send an HTML message that retrieves web bugs from a remote server. A web bug is a graphics image too tiny for a user to notice it. If the server logs the request, then you have proof the recipient read the message. Unlike return receipts, this can be done secretly. Usually you have to use a proxy (and have the service send you the return receipt) or use the service's web page to send the message and check on its status. This is the reason many email clients provide an option to disable loading remote images.
Some of these email services make more sophisticated use of HTML. For example, ReturnReceipt used IFRAMEs, bypassing most email clients' attempts to block loading remote images. Currently only a few email clients such as Kmail are able to disable that call while still displaying the message as HTML.
Some ways to defeat email tracking:
If you're trying to figure out whether a recipient read your message, there doesn't appear to be any way to guarantee it as long as you're using different mail servers. The recipient can disable a request for a return receipt. All of the tools and services for email tracking appear to rely upon the recipient not being technically knowledgeable enough to protect their privacy, that it's too much of a hassle for them to take the necessary precautions, or that they don't care.