Hi @Ben Small
For our better research, please provide more information about this case:
What is your account type (POP/IMAP/Exchange)?
Do you have the same problem with Outlook web App?
Based on the error message, make sure that the login box contains only your domain name, not your user account name. Also, if your computer remembers previous accounts, it can cause problems with login credentials. Please go to Settings, System, Accounts, check your currently logged in account. If it's a previous work account, delete it and your account information will be deleted. Then sign into your email address, open Outlook again, and try again.
Hope this helps!
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