Using dovecot-0.99.13-4.FC2
I grabbed a bunch of Mailman archive files from the Weather mailing list here:
http://www.buoy.com/pipermail/weather/
I gunzipped the older ones, then concatenated them all in correct order to create one big mbox file. I dropped it into my ~/mail folder and started my IMAP client (Mulberry) and opened the new Weather folder. It shows one gargantuan concatenated message, instead of lots of more reasonable-sized ones.
The messages appear to be properly separated by "<linefeed><linefeed>From<space>". Why doesn't dovecot parse them into separate messages?
I double-checked and removed any header lines with Content-Length, X-UID, X-IMAPbase, and X-Keywords. So this should look like a new mbox file with new content delivered by procmail. What's confusing dovecot?