First make sure that your Outlook 365 account is already setup with your domain and successfully sending and receiving emails. Here is an article that may assist in finalizing that process with Microsoft.
To setup white labeled email in Plutio with Microsoft Outlook 365
1.) Add an MX record inside your domain registrars DNS settings.
Be sure to save your changes and look for a success message!
2.) Go to Plutio Settings > Email and enter the following information:
Service: Outlook 365
Email address: Enter full business email address
Please click Save and Send to complete this process. If settings are set properly, you will get a test email, confirming that you are successfully setup!