NewsGuard refused to change one of the ratings because Mail Online did not disclose "its conservative orientation".
Despite this, the website now gets a green shield logo overall.
The NewsGuard plug-in must be switched on by users of Microsoft's Edge app, but the BBC understands there are plans for it to become the default option in the future.
The New York-based service - which is independent of Microsoft - also has ambitions to include its tool in further products from the Windows developer as well as other tech firms.
It can also be used as an add-on extension in the desktop version of web browsers including Edge, Google's Chrome, Mozilla's Firefox and Apple's Safari.