cd /var/vmail/vmail1/example.com/p/a/u/username-2005.09.20.12.30.07/Maildir
find . -type f | grep -E ',[^,]*S[^,]*$' # finds all readed
find . -type f | grep -vE ',[^,]*S[^,]*$' # find all unreaded
find . -type f | grep -vE ',[^,]*S[^,]*$' | wc -l # count them
find . -type f | grep -vE ',[^,]*S[^,]*$' | xargs rm # remove them