Problem after deleting lots of folders

On startup (and at regular intervals whilst in use), I get a popup box headed “Operations” informing me that there are errors trying to open the deleted folders. Clicking on “Ignore all” removes the popup but it keeps on returning.
Any clue how to rectify this? 

If this is with an IMAP account, it could be because there is some corruption with the cached data. You can remove the account, then add it again in Menu > Tools > Accounts.

All well again - many thanks!