Open rate is a measure of how many people on an email list open a particular email .
Often calculated as : Open rate = emails opened/(emails sent – email bounce)
Your open rate alone should not determine the success of your email blast. Although it is important to monitor your open rate along with other metrics over time.
When your email service sends out your email, they include a piece of code; which is invisible tracking code – in each one requesting the display of a tiny, transparent tracking image. When the recipient’s browser or email client tries to display the email on a screen, it reads that piece of code and sends out a request to the email service for the tracking image. This metrics is generally not accurate since it depends on clients email setting, for example, you may have subscribers with the preview window enabled in their email client, which could trigger an open even if they did not open or read the message. Similarly, some subscribers may have image viewing turned off, which could prevent opens from being tallied even though they opened and read the text in the message
The rate for opened emails will never be 100% accurate, but it is useful for comparing multiple campaigns and improving your email marketing efforts in general.