Can't delete emails from or write replies to Inbox

Hi,

I'm running Dovecot and Thunderbird, with 2 separate accounts (business and personal). One operates fine, but the other one won't allow me to write replies to the Inbox (so that I can track current threads), and if I move emails to another folder, it copies them instead. New emails do appear in the Inbox.

I've checked the permissions on the two Inboxes, and they are the same (except for the owner name). I've tried compacting the problem Inbox, but that hasn't changed things.

Edwin