I had a similar problem using courier-imap & vpopmail. Try recompiling courier-imap with '--without-authdaemon' option specified.
The incompatibility with authdaemon should have been fixed around 5.3.8. If you compile and install vpopmail 5.3.20, then recompile courier, you should not see this happen.
If you do, we'd love to hear about it. Michael Bowe and I both reviewed the code in recent versions and couldn't get it to fail in the way described in the original post.
-- Tom Collins [EMAIL PROTECTED] http://sniffter.com/ - info on the Sniffter hand-held Network Tester