Mail creating a trash folder

Please stop deleting mailboxes! What you want is Remove From Favorites. Deleting the mailbox means to delete the actual mailbox storage on your Mac. It cannot be undone...

To add mailboxes to Favorites you have to hover the mouse over the area to the right of the word "Favorites" on the left-sidebar. A tiny circle with a + will appear, click it. Looks like this:

There is an All Trash combination view. You can click the ＞ to expand any of the All entries and you should see more than one accounts file. In the case of All Trash I have three different trash / deleted items from three different accounts. You can right-click mailboxes listed under each account further down the sidebar and click Add to Favorites or Remove from Favorites but do not delete a mailbox because there is no undo for mailbox deletion.

Charles Palenz wrote:

Two days ago a folder appeared in the Mail favorites list labeled trash. Mysterious and I deleted the mail box. Turned out it apparently migrated out of my All Trash for iCloud, it's missing and I don't know how to replace it. The iCloud account has the trash folder.

Today there was another folder labeled trash that appeared when I was not looking and I searched ALL trash and each email account and none missing. So I deleted that and now it remains as a "ghost" image. Cannot be selected, but strangely it can be dragged to the top of favorites, above All inboxes or to the very bottom of All Favorites and not placed anywhere else.

̶M̶a̶l̶w̶a̶r̶e̶B̶y̶t̶e̶s̶ ̶s̶a̶y̶s̶ ̶I̶ ̶a̶m̶ ̶c̶l̶e̶a̶n̶.̶

?

You would expect an All Trash folder in mail...(?) If you spin the carrot it shows all accounts if more than one.

If you move your mouse to the right side to reveal the + you can add back in to your Favoites

If you Control click it presents you with a contextual menu—

Mail User Guide for Mac - Apple Support

macOS Upgrades can at times locate old mailboxes that were left disconnected in ~/Library/Mail and import them and they show up under On My Mac. Sometimes you'll see a Recovered Messages or Imported Messages, etc. The old email is imported because the new Mail App has changed the file storage and needs to migrate the old data to the new and it can bring in old stuff you forgot you had that was sitting there from years prior.