When you send someone a message, your hope is that the message would be opened and read.

If you know that a user opens most of the messages he/she receives, it is a good indication that yours may also be opened.

The reverse is also true. A person that hardly opens messages is unlikely to open yours.

The value of "Percentage of mail opened" shows you the fraction of mail opened by the user.

Note: If a user has 0% of messages opened, you also have to consider how long that user has had their personal ad posted. The longer the personal ad has been up, the more meaningful is the value of percentage of messages opened.