[Dovecot] IMAP message ID bad for very old message

Hi,

I just noticed that the ordering by message ID when accessing my
email via IMAP is messed up. I'm looking in the INBOX that contains
11757 emails. For some reason a load of emails from 2009 and 2010 are
being listed near the top, for example I have a message with ID 11441
from 2009 where it definitely shouldn't be, its all mixed in with
emails from 2011 there. Info from LDAP is:

• 11441 FETCH (FLAGS (\Answered \Seen) INTERNALDATE "27-Mar-2009
  09:14:10 +0000" RFC822.SIZE 5772 ENVELOPE ("Fri, 27 Mar 2009 09:15:09
  -0000" "MyC IP on CBL" (("Patrick XXXXXXXX- XXXXXXXX" NIL "patrick"
  "XXXXXXXX.co.uk")) (("Patrick XXXXXX- XXXXXXXX" NIL "patrick"
  "XXXXXXX.co.uk")) (("Patrick xxxxxx - xxxxxxx" NIL "patrick"
  "XXXXXXXX.co.uk")) (("Andy Smith" NIL "a.smith" "ukgrid.net"))
  (("Frank XXXXXXX" NIL "frank" "XXXXXXXXXXX.co.uk")(NIL NIL
  "Tim.XXXXXXXX" "XXXXXX.org")) NIL NIL
  "6937D025FA924E73B8C839858C27AEBF@PatrickDesktop"))

I'm using maildir format, and the actual file date looks fine too:

-rw-rw---- 1 vmail vmail 5627 Mar 27 2009
1238145250.H86871P67126.mx1.ukgrid.net:2,RS

I am testing IMAP via telnet (originally noticed the problem viewing
mail via HORDE/IMP). My system is FreeBSD 8.2, Dovecot 2.0.11. What might cause this behaviour?

thanks Andy.