Tracking email opens is a challenge, as the email standard does not have native support for open tracking. It is possible to send a read receipt, but these are intrusive, and only used sparingly.
The industry consensus approach to open tracking emails is to embed an invisible image in your outgoing email. When your recipient opens the email, it triggers an open.
How are opens tracked?
When you send an email, Propel adds a 1x1 pixel transparent image to your email, which is invisible to the email recipient. This image includes a unique tracking ID that can be linked to that specific email. When the recipient opens your email and downloads the image, the unique tracking ID is identified and triggers an open for that email. This entire process is transparent to the recipient, and allows for a high degree of accuracy in open tracking.
Open tracking limitations
Open tracking in this fashion has several limitations:
User must download images
Because the user must load an image to trigger an open, if the user has configured their email client not to add automatically display images, and open will not be triggered when they open your email. The default setting for most major email clients is to automatically download images, but this is not true for all email clients. Because recipients consistently use the same email client, you will notice that some contacts always trigger opens and some never do.
No way to differentiate opens between multiple recipients
Because the tracking image is connected to a specific email and not to the individual recipients of that email, there is no way to tell who has opened an email. If your email was only sent to one person, you know who opened it. If you sent your email to five people, you will know that it was opened, but not by who. Similarly, if your email is forwarded, you will not know this, but you may receive opens from multiple different people.
False positives from spam and virus filters
Some organizations have email routed through antispam and antivirus filters. These filters sometimes download attachments (including images) for the emails they scan. In cases like this, the spam or virus filters can trigger an open when the image is loaded. If this happens, you will usually see an open triggered within 30 - 60 seconds of sending your email. This could certainly be a legitimate open if your recipient is looking at their inbox when the email comes in, but it may also be triggered automatically.
Conclusions
The email standard does not provide any mechanism for tracking email opens, and so the industry standard is to include a tracking image for this purpose.
Using tracking images is a highly effective, but not perfect, mechanism for tracking email opens.
You should always pay attention to email opens as an indicator, but it is important to take this information with a grain of salt: some opens, especially those triggered within the first minute, may be false positives, and some emails will never trigger an open even if viewed by a human multiple times.