IMAP server with fast search

[Vadik <vadik@kutsyy.com>]

I run my own email server which support 5 accounts, but a few of the 
accounts have a lot of messages (4000 in the a single sub-folder is not 
uncommon), in addition to this I often need to run search for words in 
the body of the message.  Such searches are extremely slow.  I have 
setup with Maildir, and I used to run courier, but switched to dovecot 
<http://packages.debian.org/testing/mail/dovecot-imapd> (I don't really 
like courier folders system), as expected this had no effect on searching.

Any recommendation of server with good searching?

I've been thinking about a few:

Cyrus:
Also uses message per file storage, have some indexes, but I don't think 
these indexes are used for searching

UW-IMAP:
Having all messages in one file with improve search, but opening and 
manipulating such big file will slow down server.

dbmail:
sounds like a good idea, but I am not sure how stable is the server.  I 
also not sure if performance of using mysql 'like' for searching of 
message body is really big improvement.

There is a nice packages for searching archives (e.g. lurker), which are 
extremely fast, so it would be reasonable to incorporate this search 
capability in to mail server, right?

Thanks for any recommendation/comments.