In the left sidebar of Email there is an Inboxes section with all connected accounts. Click a mailbox and the inbox shows only mail from that account.
Click the same mailbox again (or click All inboxes at the top) to clear the filter. The active mailbox is highlighted in teal so you can see at a glance which filter is on.
Handy if you use different mailboxes for different clients and do not want sales mail polluting the support flow.