In the Outlook account settings for the user, the shared mailbox does not appear as an additional mailbox. This can occur for on-premises Exchange Server and cloud-hosted mailboxes in Exchange Online. Although this case was for shared mailboxes, the cause and solution apply equally to user mailboxes. In this case they were shared mailboxes and appeared in the left pane of Outlook. A customer asked about a situation in which they’re unable to remove mailboxes from users’ Outlook profiles.
