I'm observing the same problem using KDE 4.3.4. After resuming from sleep/standby/suspend, access to IMAP no longer works though POP3 is fine. Even if I manually check mail it doesn't work. Nor does it work if I manually cancel the tasks currently in operation, or click Work Offline and then go back online again. The only thing that works is quitting KMail and restarting it.