Spam is getting horrible lately. I get all sorts of new techniques including:

- using legitimate sites to bypass filters, like sending you a bill through a legitimate bill-creation site

- pretending to be a tracking service for something you supposedly ordered, then over the course of days pretending the package got lost on the way and offering a discount code for the 'purchased' amount, expecting you to use it on their phising site.

Gmail not only fails at spam classification, they classify these messages as important and nag you with first priority notifications and summaries.

The same reason spam filtering is hard. It's not possible to catch every misuse of the service without too many false positives.