First, I recommend you reading about "Read Receipts" explained in this Wikipedia page.
Some e-mail applications, such as
Microsoft Office Outlook, employ a
read-receipt tracking mechanism. The
sender selects the receipt request
option prior to sending the message,
and then upon sending, each recipient
has the option of notifying the sender
that the message was received and/or
read by the recipient.
However, requesting a receipt does not
guarantee that you will get one, for
several reasons. Very few e-mail
applications or services support read
receipts, and users can generally
disable the functionality if they so
wish. Those that do support it aren't
necessarily compatible with or capable
of recognizing requests from a
different e-mail service or
application. Generally read receipts
are only useful within an organization
where all employees/members are using
the same email service and
application.
Depending on the recipient's mail
client and settings, they may be
forced to click a notification button
before they can move on with their
work. Even though it is an opt-in
process, a recipient may consider it
inconvenient, discourteous, or
invasive.
Read receipts are sent back to your
Inbox as e-mail messages. Additional
technical information, such as who it
is from, the e-mail software they use,
and the IP addresses of the sender and
their e-mail server is available
inside the Internet headers of the
read receipt.
The technical term for these is MDN -
Message Disposition Notifications, and
they are requested by inserting one or
more of the following lines into the
email headers: X-Confirm-Reading-To:
Disposition-Notification-To: or
Return-Receipt-To:
So, if both the sender and the recipient use Microsoft Outlook with Exchange Server, they can easily request read receipts and get notified when their email is opened by the recipient.
Nevertheless, there is SpyPig.com.
SpyPig is a simple email tracking
system that sends you a notification
by email when the recipient opens your
message.
It works with virtually all modern
email programs: Outlook, Eudora, Yahoo
Email, Gmail, Hotmail, AOL Email and
many others. Both you and the
recipient must use an HTML email, not
plain-text or rich-text email.
SpyPig is FREE! No spam, no virus, no
adware, no spyware. You can use it as
often as you like, and there's no
catch.
A paid alternative to SpyPig is DidTheyReadIt.com.
When you use didtheyreadit, every
e-mail that you send is invisibly
tracked without alerting the
recipient.
But when they read your message, you
will immediately receive the following
information:
- When, exactly, your email was opened.
- How long your email remained opened.
- Where, geographically, your email was viewed.
The basic version of DidTheyReadIt is
free, but is limited to 10 messages.
If you want to continue using
DidTheyReadIt.com after this, you can
buy a subscription.
1
Related: http://superuser.com/questions/244221/detect-if-e-mail-has-been-read-by-the-recipient
– random – 2011-05-30T14:09:02.313Mephers answer is very good - Just to broaden it; You can do it your self by using an generic web handler, that serves images from your database. When you generate emails, you attach parameters
?img=234id=838
where img is the image and ID is your cusotmer ID. THe nice thing about this is you can also log IP's, times.But it requires allot of dev time. Another catch is- The email MUST download the image, or ANY other content, scripts. Which by default gets blocked now to protect users from spam machines. – Piotr Kula – 2012-03-01T16:39:35.577