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.
"Objective filters" (by which I assume you mean "rule based") can easily be worked around by the people trying to get through the filter. All you gotta do is figure out the rule and bam, you are back in again.
Sure it might work for your tiny little family domain name with your tiny 10 mailbox email server. No spammer gives a crap about you. But when you are on the scale of gmail, you will have a lot of people whose entire purpose in life is getting through your spam filters. "Objective filters" turn into a continual game of whack-a-mole. Put in a rule, wait half a day until somebody works around that rule, put in another rule.... repeat ad nauseam.
"Objective Filters" don't scale at all. They might filter out the riff-raff script kiddies but anybody whose livelihood is based on getting past your "objective filters" will easily overcome them.
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.