[Dovecot] global_script_path and virtual users

We have a big installation with postfix + dovecot + sieve. Now we would like to create a "junk" folder in every customer inbox so they can discard the spam by they own. We are planning to user global_script_path = /etc/dovecot/dovecot.sieve Where /etc/dovecot/dovecot.sieve is:

require "fileinto"; if exists "X-Spam-Flag" { fileinto "Junk"; }

My question is, will the script find the users in our database? And how, will it be using the lda configuration? I have been for weeks looking for this in the wiki and couldn't find it anywhere. I promise I'll put it in there if I find the clue :)