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Thanks for the feedback.
I understand you have deleted the Office 365 credentials in Credential Manager to force Outlook to prompt for authentication every time you open Outlook, but Outlook start to auto login again after you logon Lync. Is it right?
In my opinion, the automatic login starts again might not be caused by Lync. When you login Outlook by manually input the credentials last time, the Credential Manager may re-save the credentials for Outlook. Would you please check that does the credential for Outlook appear in Credential Manager again?
To totally disable automatic login, you may need to disable to Credential Manager Service in your OS.
Is there any update? Please let us know if you need any additional help.