Unwanted folders appeared today

I have multiple e-mail accounts with Cox Communications. On one of the accounts today in Thunderbird I suddenly have unwanted 'Sent Mail' and 'Spam' in addition to the existing 'Sent' and 'Junk' folders. Any idea what would cause this? How can I delete the unwanted folders?

The 'Sent Mail' folder has no option to delete it in right click context menu, the 'Spam' one does but it returns next time Thunderbird is opened.

Also it is concerning that this has happened to one account out of 4 with same ISP. The fact that they are involving sent and spam makes me wonder if a corrupt e-mail has done something malicious.