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Apple's native Mail app on OS X is a fitting analogy, it will also sign you in by default using the Apple ID that you signed into your Mac with, (exactly like how Windows 10 signs you into Outlook using your Microsoft account) the only difference is I can sign out of Apple Mail at any time.

Is/ was the user's PC in an AD domain? Seen this where migrated from on prem Exchange to 365 and a ghost of the old identity lives on somewhere and manages to confuse Outlook (there's a surprise!). Take a look at the machines DNS settings, may help understand. Have you run the MS connectivity analysis tool, again may help. Best James

Many departments on campus will access and manage secondary accounts for departmental messaging and collaboration purposes. While there are multiple methods to manage secondary accounts (i.e. Outlook, Apple Mail, Thunderbird, Mobile Devices, etc.), this page will step through the process to sign into two different O365 accounts on the same computer at the same time through your preferred web ...

To sign out of Outlook on the web: Select your account picture at the top of the screen. If you don't see your picture at the top of screen, check to see if ad blocking is turned on.

Click Sign in. This blue button is below the text field. Doing so will sign you into Outlook where you should see your Outlook inbox awaiting you. Before clicking Sign in, you may optionally check the "Keep me signed in" box to stay logged into your Outlook account for next time.

Sign in to my Microsoft 365 account. Sign in to access your Microsoft 365 account. Bookmark the sign-in page for easy access from your browser. Note: If you try to sign in to your account through Microsoft directly (for example, through Outlook on the web), you'll be prompted to enter your email address and then select Next before following the steps below.

Microsoft Office 365 is a cloud-based service that is designed to help meet your organization's needs for robust security, reliability, and user productivity. Office 365 Outlook | Microsoft Power Automate

If Use Cached Exchange mode is already checked, please uncheck it, save settings, restart your Outlook, check the option again, save the settings, restart Outlook and test whether the issue persists. For the next step, please repair your Outlook desktop client: Control panel> Programs> Programs and features> select Office 365> Change> Repair ...