Managing personal Outlook.com account settings, security, and privacy
Have a look at:
https://support.microsoft.com/en-us/accounts-billing/manage/microsoft-account-activity-policy
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Microsoft account activity policy: Under the MSA, you must use your Microsoft account to keep it “active.” Microsoft reserves the right to close all “inactive” Microsoft accounts.
Except as provided below, you must sign in to your Microsoft account at least once in a two-year period to keep your account active. If you don’t sign in during this time, Microsoft will consider your account to be inactive.
Any account that has been locked for more than two years will also be considered "inactive" and will be closed.
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