But does spam get by it?

Yes.Some does

Pick it up other ways.. This is about ensuring you're talking to a real mail server, and you can do some simple tests on the "batch" of mail you see in 30 minutes.

It may reduce your level of spam enough that you don't need to run something nasty like spamassassin.

Or, it may reduce the amount of crud that hits your spamassassin, and your mailservers, by a lot.