Please rename the maildircache file to something more obvious, and make it a hidden file, as in ".usermin_maildircache". Or, better yet, store this file in the existing .usermin directory in the user's home directory, instead of in the Maildir directory.
I have racked my brain (and Google's brain) several times before trying to figure out where this maildircache file was coming from while troubleshooting user e-mail problems. I didn't suspect Webmin or Usermin until recently. I searched through Dovecot source code and mailing lists, and Postfix source code and mailing lists, and both turned up nothing. It makes sense that it is for Usermin now, since I had used the Read User Mail module, but from the current file name and location, it looks more like something the mail server would use rather than a mail client file.
Also, do these files automatically get deleted at some point, or do they remain in the Maildir directory forever once Usermin has been used?