Normally, IIS would use the process identity (the user account it is running the worker process as) to access protected resources like file system or network.
With passthrough authentication, IIS will attempt to use the actual identity of the user when accessing protected resources.
If the user is not authenticated, IIS will use the application pool identity instead. If pool identity is set to NetworkService or LocalSystem, the actual Windows account used is the computer account.
The IIS warning you see is not an error, it's just a warning. The actual check will be performed at execution time, and if it fails, it'll show up in the log.
