Are you configuring your account as an IMAP type?
How do you add categories to your inbox?
As far as I know, IMAP email account types don't support categories. It's by design. More information, see: Create and assign color categories - Microsoft Support.
For accounts that support categories, such as Exchange accounts, if you set categories in your inbox, others won't be able to see them. Inbox through categories is to help yourself better manage your emails.
I actually dont think we have a shared mailbox; i believe they have just been added in with the same password on their computer...
As for shared mailboxes, it's not designed for direct logins. The user account for the shared mailbox itself should remain disabled. Please determine if there are shared mailboxes in your organization. This article can help you better understand shared mailboxes: Open and use a shared mailbox in Outlook - Microsoft Support [c757-0cb5-b08-4c11].
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