Thunderbird not displaying mails in IMAP-folder

[Benjamin Walkenhorst]

Hello everyone,

I recently set up a Courier-IMAP server (version 3.0.5) in my local 
network. I want to use Thunderbird 0.7.3 running on FreeBSD 5.2.1 to 
connect to the server.
Basically, this works. But when new mails arrive in the mailbox, 
Thunderbird only indicates them in the folder tree, when I click on the 
folder, I sometimes see the new messages, sometimes they remain invisible.
Sometimes switching to another folder in my mailbox and then back will 
help - sometimes not. Sometimes I can see the messages after some time, 
sometimes I have to restart Thunderbird.

Is this rather a Thunderbird-problem or an IMAP-problem? (Courier is 
running on NetBSD 1.6.2, if that matters - Courier's log files did not 
show any helpful messages)
Sylpheed 0.9.12 did not show this behaviour. However, I'd prefer 
Thunderbird for its ability to read both email and news.

Thanks a lot,
Benjamin