Version:           1.11.90 (using 4.2.90 (KDE 4.2.90 (KDE 4.3 Beta2)), Kubuntu packages)
Compiler:          cc
OS:                Linux (i686) release 2.6.28-13-generic

Kmail is unable to check IMAP (gmail) mail if network was disconnected while it was running.

Steps to reproduce:
Open kmail, and check for new mail.
After it is done, disconnect network.
Connect network, and try to check mail again.

It just says in status bar:
Checking account imap.gmail.com for new mail.

you can not send recive mail or open any of the folders.

Sometimes you can't simpy close kmail and star new one, you have to kill its process. I do it with Ctrl + Alt + Esc and click on the window.