I've seen a lot of recent spam campaigns that get through my basic scanning using the following tactics:
1. Careful design to not trigger Spamassassin content rules, including blocks of text to fool the bayes filter.
2, Careful omission of any identifying headers except for completely valid SPF and DKIM headers with appropriately configured DNS.
3. Real Linux mail servers dropped onto virtual hosting providers.
4. Fresh IP addresses and domains - never used domains that are not blacklisted yet and IP addresses blocks from the hosting providers that take 10-30 minutes to get blacklisted
Then they use snowshoe spam tactics to trickle them out until they're blacklisted and then move to the next domain and address.
If your address is on the lists that the perpetrators of these campaigns are using, it's really hard to avoid spam right now. Not impossible, there are some countermeasures, but vanilla Spamassassin and your standard appliances are going to have problems. I can imagine google is going to have an easier time with this because of its size and volume (=more information), but it's far from trivial.