I saved messes in the Draft folder without yet putting an email address into the to: field. These drafts could then not be deleted. In order to delete the messages I had to open the messages, enter an email address into the to: field and then I could delete them. This bug is reproducible with ease. I sometimes work on important messages and keep them in the draft folder until edited to suit. In order to prevent them being sent accidentally early while still in draft shape, I keep the to: field empty. So deleting such messages would be handy but Thunderbird requires me to put a to: email address in first.