Version:           1.5 (using KDE 3.0.9 (KDE 3.1 RC1))
Installed from:    compiled sources
Compiler:          gcc version 3.2 (Mandrake Linux 9.0 3.2-1mdk)
OS:          Linux (i686) release 2.4.19-16mdk

The following seems to happen only after I emptied some folders from a different client machine.
If a folder contains a single message and that one has status "New", it's impossible to mark it read w/o explicit Message->Mark Message->Read or shortcut. Selecting it with the mouse keeps it as new, even after I click elsewhere in the header list and then again select the message it stays as new.
Also I saw but cannot trigger now a similar behaviour:
* 2nd message arrives in the folder
* User selects the folder => first message is selected and shown as read
* User selects the 2nd message, maybe this only works with Numpad-+
* First message is shown as new again
* After another Numpad-+ hit, all is fine
As soon as the first (or first two) messages are really marked read, everything works as epected again.