There are some scenarios where you want to add a shared mailbox as the primary (or only ) Outlook email account. This means that the user does not need to change the "From" address when sending email.
- Remove any existing Outlook profiles/accounts; Control Panel > Mail
- Launch Outlook, and begin the step-by-step process to add an account
- type in the email address of the shared mailbox (not your own user email address)
- when prompted to authenticate (sign in), use your own ("real") email address and password
- complete the setup
That's it! You can also add another Exchange/Office 365 account if you want, and the shared mailbox will always be the primary mailbox