Mail Accounts No Longer at Top of List - Have I Deleted Them?

I just did something mindlessly and don't even know what is was!

My Mail app has always shown all of my POP/IMAP inbox accounts at the top of the Viewer Window. That is, all the Inboxes for my 8 different email accounts were at the top left.

Below that is the "Sent" messages organized by email account and beneath that, the folders I set up to sort read mails that I wish to save.

I just went to drag the left side of my Viewer Window to the left to widen it when I heard a 'thunk' that is the sound when I delete something. Yikes!

Now, at the top of my Viewer Window are "All Sent" categorized beneath by the various email accounts. My Inboxes have disappeared!

I must have accidentally dragged them out of the View Window ... but I have no idea where they are now! There is no 'Undo" option. I've looked everywhere.

About the only thing I can think of is to go into my backups and restore from earlier today .... but where is the data stored for the Mail app? I can't seem to find that either.

Any help is appreciated!