The "User Agent" as it is called is a widely adopted standard that email clients use to ID themselves. These strings/ID's are determined by the developer of the Email client. Some email clients have the same string for different operating systems (Gmail for Example).
Due to the user agent string, some situations may not always give accurate stats on the device or provider. Inaccurate data is less common with most major email apps and programs. The accuracy of data on some mobile platforms can be harder to differentiate due to the limited user agent data provided by the Email Application program.