[achamayou]

Can it really though? If my ISP, who I've exchanged emails with before or sends me my bills starts to send me promotional material about some TV bundle, is that spam?

Conversely, if someone I've never exchanged emails with before, but who got my details through a third party wants to discuss a business opportunity, is that spam?

If you define spam narrowly as "unsolicited", then sure, it's somewhat objective (although even then, you have to work what was solicited in ways other than an initial email). But there's quite a bit of subjectivity involved in the more useful and generally accepted definition.