macOS Mail Fix: Authorization Error 400: invalid request

Authorization Error

Error 400: invalid_request 

You cant sign in to this app because it doesn't comply with
Google's Auth 2.0 policy for keeping apps secure. You can let the app developer know that this app doesn't
comply with one or more Google validation rules.

If you are tying to add a Gmail email to Mail Application on your Mac running macOS and you get "Error 400: invalid_request" then if you read the Google troubleshooting guide page it says "Access blocked: App sent an invalid request.", which means the Mail app is using an authorization method that Google doesn’t allow.

Steps to fix this issue:

1. Login to your Gmail Account on your browser.
2. Under your profile icon and click "Manage your Google Account"
3. Select Security from the left sidebar and go to "Signing in to Google," select 2-Step Verification.
4. Go to App Password and select app as Mail and device as macOS.
5. Click on Generate
6. Now go to the Mac mail app and choose "other mail" and use this 16-character code that you generated for your device as password.