If we want to selectively delete messages from a maildir format mail account the following python script comes handy
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http://svn.houseofnate.net/unix-tools/trunk/cleanup-maildir
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An example usage i had for deleting messages older than 7 days in the Spam folder is
dir=”/the/path/to/my/maildir”
cleanup-maildir –maildir-root=$dir –age=7 delete ‘Spam’

You can use third party programs like qmail-remove http://www.linuxmagic.com/opensource/qmail/qmail-remove to accomplish queue management in plesk control panel managed qmail
The Plesk control panel itself provides a modified version od qmHandle ( http://qmhandle.sourceforge.net/ )along with it that can be used with ease to manage the queue
To see the statistics of the queue:
–
/usr/local/psa/admin/bin/mailqueuemng -s
Messages in local queue:  0
Messages [...]