Read rates: Return Path reports an email as Read when your subscriber selects an email and its status changes from Unread to Marked as Read. However, Read rates do not take engagement or whether images were enabled or disabled into consideration. This data comes directly to Return Path from mailbox providers.

Open rates: ESPs use a small image called a tracking pixel or web beacon to identify a message as open. The tracking pixel is typically a transparent image that is embedded in the code of HTML emails and usually includes a unique identifier to track which subscriber opened which message. For spammers, this information is highly valuable as it means that this person can be targeted and their email address can be sold. This is one of the main reasons why images are typically blocked by default at mailbox providers.

The difference: Marketers that send text-only emails cannot get open rates since you cannot embed an image in a text email. Marketers that send HTML emails cannot get open rates on emails when images are not enabled or displayed. The email client must display an email's images for an open to be registered.

Therefore, the main difference is that the Read rate is able to capture information about a subscriber opening their email regardless if their images are enabled or disabled. Open rates may be less reliable since there are subscribers and mailboxes that have images disabled by default.