[exim] .procmailrc to move messages from folder

Hi all,

i would like to configure a .procmailrc file in the [...]/Maildir
folder of each user in order to move from the .Inbox to .Spam folder
those messages that start in the subject with the string [SPAM?]

I've never seen a procmail script and the test i have been doing seems
no to be good enough. I would like to get from the communitiy some
examples.

TIA,
BR,

jonathan