[Dovecot] strange email client problems

[Gabriel Granger]

I've got some issues with dovecot, that I need to resolve, I only have 
about 20 users in the office, but the average disk space used by each 
user is anywhere from 1GB up to at the extreme end of the scale 7GB.  
I'm using the mailbox format.  The server is a P4 2.8 with a 1GB of ram 
and run off 250GB SATA disks.  most users have lots of structured 
folders going as deep as 6 levels.  The problems I'm getting are odd

the problems that I'm getting at the moment are

"cannot copy mail to sent folder", but sends the message fine, this 
doesn't happen all the time but happens to everyone but me... I'm using 
OS X's Mail.app which seems to always work:) all my users are using W2K 
and either ThunderBird or Mozilla

The other problem which I've been made aware of...

The users get the "loading message" or "opening messages" and the egg 
timer goes round but never completes.  I managed to clear the problem 
by getting the user to close their email client and then kill their 
IMAP processes on the server, when the user logs in again the client 
responds correctly.

Is there something I'm doing wrong with the configuration of dovecot? 
would using MailDir help, I dont want to migrate from Mailbox unless 
this change will make things better, I understand their are benefits 
from using MailDir, but I dont know if I will continue to see the same 
problems if I migrate.

Any help or advise would be great, I should add that everyone is 
connecting to the server over SSL

Gabe