[Dovecot] Email being stored in "cur" folder and not being downloaded in Thunderbird

Hi,

I have recently noticed that a random number of emails are being stored in the "cur" folder under certain users and cannot be downloaded in Thunderbird. I have tried moving the emails from "cur" to "new" but everytime I try to download the email from Thunderbird, the emails go back to the "cur" folder. What could be causing this?

What is the "cur" folder used for? I had thought that the "cur" folder was used for IMAP but all the users here are using POP.

Also, I have tried testing this on Outlook and Outlook seems to download the files from "cur".

Thanks