What causes folders to be reported as noselect?

As the subject says.  This may be a bit open-ended - but it would really help troubleshooting some obscure folder issues.

In my case, I happen to have both some "real" folders and also some "virtual" folders that respond to IMAP LIST commands with the "\NoSelect" flag - and I don't know why.  Via telnet, I can manually issue SELECT, SEARCH, and FETCH for such folders without errors.

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Daniel