Wonderful job you are doing, Jamie.
Webmin 1.470, Sendmail 8.13.8 on CentOS 5.
The "Flush Selected Messages" does not work, if the message was quarantined. Need to be able to do that in case of a false positive. I hacked my del_mailqs.cgi file so that flush uses:
$cmd .= " -qQ -qI$file";
The -qQ is needed to tell Sendmail that the file is in the quarantine queue. Otherwise, sendmail just sits there (sort of like MSExchange does normally).
Granted, I am sure that you have a more elegant way to check for status and adjust the command accordingly. Mine was just a quick fix. "Flush Mail Queue" and "Flush Quarantined Queue" work just dandy.
Again, your hard work is truly appreciated.