[Dovecot] mbox separators ignored

[Kenneth Porter]

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?