All inboxes option has disappeared from ‘Inboxes’

Now hover your mouse over Favorites and click the + icon. It will show you the following screen:

From the Mailbox to Add, select All Inboxes from the drop-down list and click OK.

I also have this question, but I don't see a "< Mailboxes" in the top left of my Mail app. My OS is 11.2.3. Strangely my "All Inboxes" has disappeared and my mail accounts have moved to the bottom of the grey mailbox column. I only have Favourites, including "VIPs", "All Drafts", "All Sent', " Flagged", "On my Mac" and then, at the very bottom, (they used to be at the top where I prefer them), my different mail accounts. How do I get my "All Inboxes" back and my accounts to the top of the column? Thank you for your help.

The solution was actually for an iPad, not a Mac which I surmised you have since you mention "OS 11.2.3" which is Big Sur.

In your case, in the Mail app, hover your mouse over Favorites in the left side panel. I think you will see this icon: ">". If so, click on it: this will expand your mail subfolders which include the "All Inboxes".

If you see the down arrow instead (meaning your Favorites folder is already expanded), click the + icon and follow the prompts to add back your "All Inboxes".

Someone, (you?) sent me an image and I now have my "All Inboxes " back. Thank you! Now, is there any way I can move my mail accounts back to the top of that column, where they used to be?

I just solved my own problem, so thank you for your help.

Depending on what the Mail app is showing, see if there is a "< Mailboxes" in the top left of the screen and tap on it. Now, if you see "Mailboxes" in the left pane, tap Edit and make sure "All Inboxes" is selected and tap Done.

This is what I see with Favorites expanded:

Yes, you can drag the folders in the Favorites list to any position you prefer. Click on a folder and move your mouse to where you want the folder to be located.

Great! I'm glad I could be of some help.