Do you use KDE's mail user agent KMail? Do you use it with IMAP? If you do, you may be aware of various error messages such as ‘Unable to stat resource’.

For the benefit of people looking for a solution using search engines, the top few lines of the error message read something like this:

Error while getting folder information.

Could Not Determine Resource Status

An attempt to determine information about the status of the resource INBOX.customflags,
such as the resource name, type, size, etc., was unsuccessful.

Technical reason: Could Not Stat Resoure

I, and everyone at my company using KMail on our Courier IMAP server, used to get this error message every time KMail checked for new mail. This was a mere annoyance for a while, but eventually it became to much and I set out to debug it. I looked for a solution online and didn't find one. Armed with my trusty wireshark, I watched the client-server dialogue for clues as to what was happening with the .customflags folder, and found the issue.

On our mail server, .customflags is a file storing the custom mail flags used by mail user agents for a particular maildir. KMail seems to be unaware of this file's special purpose, and treats it like a mail folder — which it isn't. Cue errors.

Solutions

There are a couple of solutions which work in different cases. Please bear in mind that I'm by no means an IMAP guru. These solutions are empirical, and I didn't check the RFCs for their validity. Feel free to tell me I'm a cretin and suggest a better solution!

The obvious way out of this issue is to delete the .customflags file from the server's Maildir. You can use a shell account, or try deleting it via KMail (I can't vouch for this way of doing it, I prefer using the shell).

Initially, I attempted a fix where I recreated .customflags as a proper mail folder, thinking perhaps KMail uses it to store its custom flags. That worked too, but I didn't see any difference.

Also, you can enable subscriptions on your IMAP account, and make sure the .customflags entry is permanently unsubscribed. Right-click on the account (on the account sidebar), select Subscription..., find .customflags by expanding the folder tree (or using the search bar), make sure it's not ticked, and click Okay. If subscriptions are disabled, you'll be offered to enable them. Agree to this. With subscriptions disabled, KMail will look through every folder whenever it checks for new mail.

Good luck!