Hello, thank you for reaching out to Microsoft Q&A.
It appears that you received a legitimate email notification from Microsoft.
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Since you mentioned that you no longer use OneDrive, could you please check whether you are signed in with your @gmail.com account on the OneDrive desktop app or on your mobile device?
To check on Windows, right-click the OneDrive icon in the taskbar, then select Settings > Accounts.
If the account is linked, it is possible that your files are still syncing to the OneDrive cloud, which may be using up your storage space.
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Please go to https://onedrive.live.com/ and sign in with your Microsoft account.
Check whether you can locate any files that were synced from your computer.
If you find them, first back up those files to your local drive.
Then, free up space in your OneDrive account by deleting files you no longer need (after confirming they are safely backed up locally). Also, delete them from your OneDrive Recycle Bin folder.
Finally, unlink your account from the OneDrive desktop app and uninstall OneDrive if you no longer wish to use it.
Let me know if you still need more help.