With IMAP you can mark emails as deleted and some email clients will not even show them any more. But the emails are still there and occupy space. Usually there is an option to purge all marked emails but many users do not care. So Michael Weisgerber suggests to run this command frequently via crontab to remove such emails:

$> find /var/vmail -type f -ctime +7 -name '*,ST' -print0 | xargs -r -0 rm -f

Dovecot renames all deleted emails so that they get a ,ST added at the end of the filename. Adjust the parameter to -ctime as you like. In this example deleted mails older than 7 days are purged.