Microsoft users who have received "Your OneDrive is Full" emails like the one below are asked not to click on the links in them. This is because the fake emails are phishing scams that go to a form created at forms.office.com by cybercriminals that steals account credentials.
If you want to know if your OneDrive is full, always go directly to onedrive.live.com and sign-in from there, instead of clicking on a link, especially one in an email message.
A "Your OneDrive is Full" Phishing Scam
Security Info <email@example.com>
Thu 10/3/2019 1:00 PM
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Orange caution triangle
Your OneDrive is full
Your @hotmail.com personal account is out of space. To keep using OneDrive, you can upgrade your account to get more storage or remove files you don’t need.
While your account is full, you won't be able to add or sync new documents or photos to your OneDrive. Also, any changes made to existing OneDrive files won’t sync between your devices or OneDrive.com.
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The Fake "Your OneDrive is Full" Form
Update: Microsoft has blocked the form at forms.office.com because it is asking for personal information: