"Drop in" replacement for imap-UW?

[Joe in MPLS]

I've been running imap-UW on FreeBSD 6.x STABLE for a while. Clients are 
mostly Thunderbird, TREOs and Horde (running on the same box.

I'm having issues when multiple clients try to access the same user's 
mailbox. Typically the TREO will do a scheduled check for new mail while 
Thunderbird has it already open. Research indicates that this is a 
limitation of the UW imapd implementation.

Can anyone recommend a good and secure imapd implementation that won't 
require a ton of reconfiguration? Mostly I'd like the replacement to be 
able to use all the existing end-user imap folders and mail spools. All 
users have accounts and mail spools on the box. There's no virtual users 
or database authentication backends to deal with.

        TIA     ...jgm