When sending to multiple recipients, Bananatag will not be able to tell you for certain which specific contact has triggered each open. This is because when putting multiple people in a message (either in the To:, CC:, or BCC: fields) they all receive the exact same email, which means there is only one tracking code used.
Many times however, you can tell who opened an email based on the location/device information that you get from the open in your Bananatag Account.
Bananatag was configured this way to work in line with existing email workflow. (For example, if a separate email was sent to each recipient it would make it difficult for users to reply all.)