@Marcel Just
How POP3 works, each user email app (for example Thunderbird, Outlook, etc.) keeps track individually, locally, of the "newest" email message it has received, and which emails messages are "read" or "unread". Each user would have a personalized experience of what is the new email messages, and what is read/unread.
For IMAP, you are right, it is a shared email inbox, and as soon as anyone reads a message, it would appear modified (read/unread).
I think you're missing the point of Odoo messaging, relative to shared incoming email inboxes. For example, the inbox support@yourcompany. You probably don't want tthose messages coming into many employee inboxes. You probably want Odoo to automatically create a new record of type Helpdesk (crm.helpdesk), and have the Helpdesk record automatically assigned to whichever group of emplyees is intended to handle the customer helpdesk.