[Dovecot] Mailbox isn't a valid mbox file

Getting "Mailbox isn't a valid mbox file" on 1.2rc2 after transferring mail from an ancient 0.99.13 system. The first line of the file looks ok. Curiously, it's one of my archival folders of this list. Here's the first header from the top of the file:

From tss at iki.fi Sat May 1 21:48:17 2004 From: tss at iki.fi (Timo Sirainen) Date: Sat May 1 21:54:18 2004 Subject: [Dovecot] 1.0-test2 released Message-ID: 1083437297.21296.19.camel@hurina X-IMAPbase: 1085322222 20128 Status: RO X-UID: 14065 Content-Length: 226989 X-Keywords:

It would be nice if the code that verified the mailbox format would log details about what it disliked.