Server Side Spam Filter

Question

This should be easy, but I have spent hours looking into this and can't find an obvious way to do this.
I am running OS X Yosemite 10.10.1 and OS X Server 4.0
What I want to do is setup something on the server that will perform a redirection of mail, that is marked as Spam, into each users Junk folder so that they can then decide what to do with it without it filling their inbox.
I have my own domain and manage the mail accounts for my family, so it is a very small user community that I have to manage.
On the iMac and MacBook this is easily done in the mail app, through the Junk Mail options, but both my children generally use their iPads to access mail and this feature is not available on the iPad and iPhone.
As they both spend a lot of time on Facebook and other social network there is a large amount of undesirable Spam mail coming through, it gets marked as Spam, but my children still then have to deal with it. If I could get it automatically moved to the Junk folder they would only have to go there if a mail that they were expecting hadn't arrive and had inadvertanly been marked as Spam.
Reading the OS X Mail Service Architecture page, it looks like this should be relatively easily done via Dovecot, but I have seen two conflicting methods for doing it.
One that says I can set a global variable in the dovecot.conf file, "sieve_global_path = /var/vmail/globalsieverc", but this variable is not in the default OS X version of dovecot.conf, and another one that says you have to modify each users configuration files separately, which just seems wrong to me.
As I have already tried a couple of methods, with disastrous results, I am reluctant to keep trying random solutions.
So I am reaching out to this community to find out if anyone here has successfully setup this type of filtering

Answer 1

Hi Mike,
I have a solution and can confirm all is working well.  This is how to do it...
1.) Create a master sieve config file
sudo mkdir /var/lib/dovecot/sieve/
sudo vi /var/lib/dovecot/sieve/default.sieve
In that file, add:
require ["fileinto"];
# Move spam to spam folder
if header :contains "X-Spam-Flag" ["YES"] {
  fileinto "Junk";
  stop;
sudo chown -R _dovecot:mail /var/lib/dovecot
sudo sievec /var/lib/dovecot/sieve/default.sieve
2.) Add the SPAM sieve conf include to the main config
sudo vi /Library/Server/Mail/Config/dovecot/conf.d/90-sieve.conf
Add the following (you can use the existing sieve_before and modify it):
sieve_before = /var/lib/dovecot/sieve/default.sieve
3.) Ensure that the Junk Folder is automatically created and subscribed to by default
sudo vi /Library/Server/Mail/Config/dovecot/conf.d/15-mailboxes.conf
Replace:
mailbox Junk {
    special_use = \Junk
With:
mailbox Junk {
    auto = subscribe
    special_use = \Junk
4.) Restart all Mail server services using the Server.app
5.) Ensure that SPAM is going into the Junk folder correctly.  You can test here: http://www.emailsecuritycheck.net/