That was my bug report, Bug #454:
http://www.Virtualmin.com/bug-tracker/bug?bug_number=454Note that I was not able to convince Jamie to move this file out of the Maildir folder, because there was not enough evidence at the time that it interfered with IMAP servers. However, your message may change that. In the meantime, Jamie made the file hidden and renamed it to .usermin-maildircache to make it's purpose more obvious. There is no option to move the file, because we both agreed it would rarely get used and only make things more complicated.
BTW, you can search the entire Virtualmin site, including the bug tracker and the forums, by doing a Google "site" search:
http://www.Google.com/search?q=site%3Awww.virtualmin.com%20maildircache