Why Gmail refuses your normal password
Google does not let outside mail apps sign in with your regular Google password. An outside app has to use either Google's own sign-in page or an app password, a 16 character code made just for that one app. If you typed your usual password into a mail app, Gmail rejects it every time, however correct the password is.
Why an app password stops working
App passwords only exist while 2-Step Verification is turned on for the Google account. If 2-Step Verification is switched off, or you change your Google password, the app passwords you made earlier can stop working and you need a new one. Google shows the code only once when you create it, so a code that was copied wrongly, or with a stray character, is refused as well.
Work and school Gmail accounts can be different
If your address is on a company or school domain that uses Google Workspace, your administrator controls whether app passwords are allowed. If the App Passwords page is missing for you, that is an admin setting, and a new code cannot fix it. Ask your administrator to allow app passwords, or to approve the app you are trying to use.
The fix: make a fresh Gmail app password
- Make sure 2-Step Verification is on for the Google account. App passwords do not exist without it.
- Open Google's App Passwords page, name the app Curo, and click Create.
- Copy the 16 character code Google shows you. You only see it once. Spaces in the code are fine.
- In Curo, add a mailbox and type your Gmail address. Curo recognises Gmail and fills in the server settings for you.
- Paste the code into the app password box and click Connect and sync. Curo starts sorting your mail right away.