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The current command did not succeed. The mail server responded: RENAME failed: Can't create mailbox node /home/pop/account31/Trash/: File exists.

IMAP folder cannot be deleted
From MozillaZine Knowledge Base

When you try to delete an IMAP mail folder in Thunderbird (or Mozilla Suite), you may find that you cannot delete the folder and that you instead receive an error message that says "The current command did not succeed. The mail server responded: RENAME failed: Can't create mailbox [folder name]: File Exists."

In this case, try the following:

    * Thunderbird users: go to "Tools -> Account Settings -> [account name] Server Settings" and change the option for "When I delete a message" to "Remove it immediately."
    * Mozilla Suite users: go to "Edit -> Mail & Newsgroup Account Settings -> [account name] Server Settings" and change the option for "When I delete a message" to "Remove it immediately."