When I sort the inbox on any of the columns (from, date, size, subject) there appear a lot of e-mails that, according to the page you get when clicking on them, no longer exist.
When using the "Reset sorting" function the inbox turns back to normal.
I found a similar report on the Virtualmin forum: http://www.virtualmin.com/node/10223
There is also a screenshot posted there.
It seems like for every mail that is deleted over IMAP protocol there is left such a message with From: "None" and Size: "0 kB".
I am running Virtualmin, Webmin and Usermin all updated to the latest version on Ubuntu server 10.04 (also up to date).
When I go to Webmin > Postfix Mail Server > User Mailboxes > username I will find the same problem.