Several Inboxes

I would like each account to have its own Inbox but can’t find how to do it.

It’s automatic, As you add new accounts, it creates a separate dropdown for that email account. You’d just need to expand them so you can see the individual inboxes.

When adding IMAP or Exchange accounts, they will always have their own Inboxes.

POP3 accounts when setup, by default, have their own Inboxes as well. However, you can specify during a manual installation of the account, that they share a common Inbox.

There is unfortunately no way to undo that setting at a later stage. The only solution is to setup the account again, making sure that you have selected separate folder trees.