I'm using Thunderbird (various versions) on all my computers.
I have set up filtering on one computer that's always up, so that eg mail from "abc.se" is delivered to folder "Abc" under Inbox.
This works fine, but the problem is that on my other computers I can't see if there's new mail when it's been filtered. Normally, when new mail arrives unfiltered, Inbox is bold and there's a number after showing amount of unread mails.
But when mails are filtered nothing is shown on other computer until I click on the subfolder.
Any ideas how to fix this?

It does not matter what version of Thunderbird or OS - used Debian Lenny, LMDE, OpenSuse 12.1, Windows 7, same on all.

It is due to the client reading and moving the email so your server thinks it is read. Solution: Don't filter or check the folder manually.

Well, that makes sense - but it also makes me very sad!
Is there no way to get Thunderbird - or any mailclient - to automaticaly mark mails unread?
Guess not, but it's a question worth asking!

Second thoughts - no, this is wrong.
Mails moved by a filter rule are *not* marked as read.
But mail filtered by Thunderbird on one computer don't show up as unread on other computer before I click on the folder.

At last I found the solution!
In Thunderbird, Advanced configuration, locate "check_all_imap_folders_for_new" and change it to "true".