How do I open another mailbox in Outlook 365?
Note: If you do not have the option to Open another mailbox use this URL in your address bar in the browser window: https://outlook.office365.com/mail/username@domain.com. You will replace the email address with the part that says username@domain.com
Once you start to fill out the email address it may auto-fill for you.
Note: If the mailbox doesn't auto-populate select Search Directory