Most of my mail is delivered by smtp directly to me, but my
secondary max records cause some mail to be delivered to a POP3
mailbox- this is then retrieved using spamassasin.

This works very well generally - I get my mail very quickly and implement spam/virus filtering.
The only problem is that many spammers deliberately target secondary
mail servers as being less likely to have spam filtering, so I still
filter mail received vi fetchmail even though this is seen by my mail
server as local mail.

Having been running spamassassin at home with great success for some time, I have just set it up at work but had a few problems setting up a new Debian Linux box for this from scratch.

I found that spam was just not being detected nearly as well as at home, and the machine was struggling with the load (load average around 3).

1.

I noticed that the blacklists were not being used,
after a bit of digging (wiki.apache.org/spamassassin/DnsBlocklists I discovered I needed to install the package libnet-dns-perl