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