If you right click on a unified folder, click properties, and then click choose, do you then see all the accounts?

I don't know if I understand you well. If I right-click on a unified folder, I don't see PROPERTIES (Proprietà). I attach a pic.

It appears you did a screenshot from All Folders, not Unified Folders. once you turn on unified folders, click the 'Inbox' folder, rightclick and then select properties and then click 'choose'

Now I understand, I thank you. YES, I see all my 20 accounts. In that panel I see an "Aggiorna" button. Now have I to click on "Aggiorna" (Update)?

Yes, click update to save the new settings. I'm glad all is working. :) You are very welcome,

I now clicked the "Update" button, but nothing has changed. I always see 14 Accounts with unified folders, even if on "Choose" I see all 20 Accounts. The "Update" button didn't work.

You have tick marks on all twenty accounts? And when you press 'update', nothing happens? That is a lot of accounts, but should still work. Something else to try: '

1. click help>troubleshootinginformation'
2. scroll to 'profile folder' and click 'open folder'
3. exit thunderbird so it will be closed
4. click the Mail folder
5. you should see a folder named 'smart mailboxes'. DELETE IT
6. restart thunderbird and check that unified folders are working and repeat steps from earlier response to put tick marks on all twenty accounts. NOTE: other accounts will not display in unified unless their folder is the same name as used in other accounts. For example, a unified folder for inbox will only show accounts that have a folder named inbox.

Unfortunately none of the methods work. 1) I have a tick marks on all 20 accounts and when I click on "Update" nothing happens. 2) In help>troubleshootinginformation, after opening the folder, I don't see "smart mailboxes". In fact, I deleted it a few days ago because I had read this method on the Internet. But nothing happened and nothing is happening now. I always have 14 accounts in "Unified Folders". I'm very sorry about this. In fact, if I can't see them all, maybe I'll change my email client. I would be sorry!!

Can you post a screenshot of the Mail folder? I am puzzled on this. Thank you.

Sure!! There it is!!