Return Path defines Unspecified in Email Client Monitor as data that could not be translated from the user-agent string and is too vague to be categorized.
For example, there are some instances when mailbox providers update or change the user-agent string which then causes emails to be marked as Unspecified. This happens until Return Path is able to update the system to read the new user-agent string.
User-agent string definition
A user-agent string is a text string that is a part of the client-server conversation. The user-agent string is managed by the mailbox provider or browser. Once the Email Client Monitor pixel is triggered, the pixel sees the user-agent string and gathers that data for Return Path. Return Path translates the user-agent string to determine the email client and platform a subscriber is using.