Mail not allowing me to delete mailboxes/folders

Hi All,

I have many folders/mailbox's setup in Mail, I always right click on a folder and select delete to delete the folder but when I've tried recently I hear the error sound and nothing happens.

The same happens when I try to create a new folder as well, the error sound plays and nothing happens...

Has anyone experienced this before?

Mail seems to be giving me a lot of trouble since upgrading to Mountain Lion...