How do I define server-side spam rules with Yosemite & Server 4

Question

Really surprised that this isn't functionality available in the server UI. Poking around in the dovecot/spamassassin docs imply this is possible using sieve, but the configuration looks more complex than I'd like to have to figure out on my own. It seems that sieve might be possible to use. In /Library/Server/Mail/Config/dovecot/conf.d, the option sieve_dir is uncommented, pointing to /Library/Server/Mail/Data/rules/%u. That seems to imply I could create user directories there and apply some sieve rules, has anyone tried this?
I'd rather not do user-specific rules, I'd prefer to use the default and attempt to move all spam into a spam folder for each user.
I've seen notes that the webmail client Roundcube provided server-side rules, but I really don't need the web mail client, and various search results seem to raise questions about whether Roundcube is compatible with Sever 4 / Yosemite.
Any help would be appreciated...

Answer 1

Was able to get it working with some digging. The basic summary is that you need to create a rules directory for each mail user, and then add a sieve rule in each of the directories you create. I did this manually, which is manageable because I'm using OS X server for my household, and we don't add members to my household that often... :-). Summary of the steps:
1) Each user has a directory in /Library/Server/Mail/Data/mail that is in UUID format (long hex string with sections broken by - characters).
2) For each of the users you identify in (1), create an equivalent UUID directory in /Library/Server/Mail/Data/rules
3) Create a file named "dovecot.sieve" in each of the directories you created in (2). For a basic rule that moves mail marked by SpamAssassin as spam to the Junk folder that Mail.app should create by default, you can use the following:
     require ["fileinto"];
     # rule:[SPAM]
     if header :contains "X-Spam-Flag" "YES" {
             fileinto "Junk";
4) Ensure that all of the directories/files are owned by _dovecot. I marked dovecot.sieve as executable, but I'm not sure that was necessary.
5) Once you've done steps 1-4, you should start to see evidence that sieve is being used for delivery of mail in /Library/Logs/Mail/mailinfo.log. Grep for "sieve" in that to see activity. If there are errors, you'll likely see notices that there are sieve logs being created in the rules directories you set up in (2). I ran into errors for at least one account that didn't have a Junk folder created; if you hit that just have the user create a folder named Junk to clear it up.
Hope this helps others...