Are you using maildir? AFAIK, various imap servers add extensions to 
maildir, so if the mailservers involved use different imap 
implementations you could run into problems there.

Miles O'Neal wrote:
> We have an odd email setup.  Our primary, internal
> mailserver is an old Solaris 9 system running postfix,
> imap and pop.  This system is really getting bogged
> down and we plan to move mail to another system soon.
> But short term we have started running imap and pop
> services via xinetd on two SL304 systems to offload
> the main mail server AMAP.  All three servers mount
> the mail directory from the same filer (NetApp) via
> NFS.
>
> For most users, changing the imap or pop server in
> the client just works (as it should).  But a handful
> of users, most running Thunderbird on Linux (a couple
> running default clients on Mac or XP laptops) have
> found that when they switch servers, they see an
> empty inbox, or only a handful of the most recent
> emails there, no matter how many times they try to
> sync.  If they switch back to the slow, original
> server, everything shows up again.
>
> The desktops are running 4.4 and some version
> of Thunderbird 1.5 or 2.0.
>
> Have any of you seen anything like this?  Is it an
> imap (and pop) problem on EL3, or a protocol bug,
> or what?  I don't see it as a thunderbird issue
> since we have a handful of Mac and Windows users
> seeing it on clients that came with their MacOS
> or XP.  Of course, weirder things than that have
> happened...
>
> Thanks,
> Miles
>