I'm not sure I like the use of the word bulk. The reason is that it is not precise. Is 10 bulk? 50? Is it only bulk if I use a "spam tool"?
Bulk is more than 1 copy. How do I know if something is bulk? A simple test. Is this something that could have been sent to someone else with either no modification, or a trivial "mailmerge" operation. It then becomes up to the spammer to prove otherwise to his abuse desk, who will probably have received multiple complaints anyway.
I generally measure bulk in a more subjective but more useful way. If someone composes ten pages of text and sends it to three people, I don't consider that bulk. If someone sends one paragraph of text they composed to fifty people, that's bulk. If someone ads 'look at this' to twenty pages they stole from someone else and sends it to 10 people, that's bulk. The test is, is this person trying to spread a minimum amount of original content to the maximum number of people? Or is the content specifically targeted to each person by a human being? In other words, is this a rifle being aimed or a machine gun being sprayed? Is a person trying to use email as a publishing means? I have no objection if someone who honestly saw a message I wrote and thought I'd be suitable for a particular job emails me asking if I'm interested. However, the same email would be bulk if sent to everyone who posts to NANOG, even if says, "I saw your post about "Re: Fwd: Re: Digital Island sponsors DoS attempt" and thought you might be interested in buying our premium fishing worms". DS