Hi @mcttsun
Welcome to our forum!
Please kindly understand that the Outlook tag here we mainly focus on general issues about Outlook desktop client. The issue may be related to the account itself, it's recommended to contact the mailbox admin and account support directly to get more help.
Is your account personal or organizational?
As far as I know, you can manage your security information here: https://go.microsoft.com/fwlink/?linkid=2204804.
On your Advanced security options page, you can review existing sign-in methods or add a new way to sign in or verify. More information, see: Microsoft account security info & verification codes - Microsoft Support.
Also, notice that you are identified with unusual activity. In light of this situation, you can see when and where you accessed your account, including successful logins and security challenges, on the Recent Activity page. Microsoft understands how you typically sign into your account and flag suspicious events.
At last, for the security of your account and property, please follow the steps in this document to protect the security of your account.
How to help keep your Microsoft account safe and secure - Microsoft Support.
Hope this helps!
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