There are many types of emails which you receive but will never actually reply to. For example, a client may email "Sounds great - please go ahead." These kinds of emails should be kept as a reference for the future.
Emails that came into your mailbox should not be left in your Inbox. The aim is to read, action (if needed) and delete. You should be trying to get your Inbox down to 0 items.
So what's left in your 'Inbox' should only be 'To Do' items. Sure you might want to add subfolders to group related projects etc. but these subfolders should also contain items 'To Do'.
Leave emails in the Inbox folder for later reference is not a good idea. You should create folders outside your Inbox for such emails.
✅ Figure: Good example - Save important reference items in a separate folder
Microsoft Outlook provides you with six main folders:
But we believe they are missing two additional folders:
You can use these extra folders to store work-related or personal emails that you wanted to keep.