Version:            (using KDE 4.2.0)
OS:                Linux
Installed from:    Fedora RPMs

I have KMail setup with 2 online IMAP account and 1 offline IMAP accounts.

Observed:

1. /home was 100% full.
2. I launched KMail.
3. KMail showed several error messages (very sorry - I didn't record it exactly): "Can't save UID cache file for mailbox X", each one for a different folder.
4. KMail did not start - the KMail window did not open.
5. I cleared some space from /home.
6. I launched KMail again.
7. The KMail window opened.
8. KMail showed a dialog saying this was the first time I had run KMail (it wasn't).
9. Account settings where intact (the three IMAP accounts appeared in the folder tree).
10. Other settings (e.g. the threading mode for each folder) were lost.
11. The Unread Mail count for the folders was incorrect (they were all displayed as having no unread mail).
12. After clicking on one of the folders I noticed it was missing some of the messages that should have been in it.
13. The missing message headers were re-downloaded from the IMAP server.
14. This problem was also evident in other folders - one folder was missing over a year's worth of messages.

Expected behaviour:

/home was 100% full. I launch KMail.
1. KMail shows a warning: "Can't write UID cache file for mailbox X - disk is full."
2. KMail window opens.
3. When trying to download new messages KMail displays a warning: "Can't download messages - disk is full."
4. No settings or already downloaded messages are lost.