What does 'unknown' platform mean?

An Open being registered in the 'Unknown' platform means that we do not have enough information to categorize the open into another Platform category.

There are many reasons why an open can be counted in the 'Unknown' platform. The client's settings may prevent required information from being returned to us, or they could be using a mail client that the system does not support and/or recognize.

This platform information is commonly found in the Mailing Overview report, though it is possible to see platform information when digging into open and click lists from sent campaigns.