If you delete all messages from mailbox, INTERNAL DATA message doesn't show up there? The mbox file size becomes zero? If so, it doesn't sound like you're really using Dovecot v1.x..
It looks like if I'm starting without the folder existing (whether it's a new delivery rule or you delete the folder from IMAP), deliver doesn't create the FOLDER INTERNAL DATA so it will just start appending messages to it.
Once that first message is received, IMAP won't create the FOLDER INTERNAL DATA message until I delete all the messages from the folder.
When I delete all the messages from the folder, they are not actuall removed until I restart my mail client. If new messages are received to that folder before I restart my mail client, then it also won't create the FOLDER INTERNAL DATA message.
I'm not sure if that's directly related to this issue, but I know the corrupted From line is always the first message in a mailbox and is never the FOLDER INTERNAL DATA message.
