A while ago I was getting some spam that would not go away. I found that if surbl was configured on spamassassin then I would not have been getting the email.

In spamassassin 3.1 they are installed automatically. If you look in
/opt/zimbra/conf/spamassassin/25_uribl.cf

you will see the settings for surbls.

From a console if you run the command

spamassassin --lint -D

You will see a bunch of output.
Look for URIDNSBL in the text somewhere, I think this mean that it's loading and doing the surbl's.