David Baker, a global vice president at marketing technologies firm Acxiom and e-mail expert extraordinaire, wrote recently that when speaking with an employee at one of the major online database aggregators, he was informed that this company had a grand total of 4 billion e-mail addresses on file.

And of these, ~2 billion had names and addresses associated with them.

These numbers are dramatically higher than the worldwide estimate of e-mail addresses published by the Pingdom blog in its Internet 2011 in Numbers report.

Think about this for a moment. Considering that the total population of the United States is a little over 310 million, how many e-mail addresses per person are floating around out there?

Strip away the very young … plus teens and ‘tweens who don’t engage nearly so much in e-mail … and we’re left with the realization that among the core adult audience of Boomers and GenXers, there’s really no such thing as a single e-mail address that can be tied to one individual.

Even if we ourselves don’t maintain multiple e-mail accounts for different purposes, surely we know people who do. One person I know is juggling no fewer than 20 separate e-mail addresses; he claims to be keeping them all straight.

This notion of multiplicity is at cross-purposes with how marketers have traditionally viewed prospecting. We’ve been conditioned to think about an individual as being tied to one physical address and one e-mail address – in the same manner as a discrete mobile phone number or a unique social security number.

In theory, all of these are vehicles of monetization, with e-mail being particularly attractive because of the low cost associated with reaching prospects in that manner.

But in actuality, there’s a great deal of complexity:

• Which e-mails are associated with opt-in permission?

• Which e-mail addresses are primary (highly active) versus secondary (relatively inactive)?

• Which e-mails are valid, but lying dormant?

Because e-mail addresses are “cheap/free,” they’re ephemeral. They aren’t “linear” in the same sense as the data on a residence, a business address or even a mobile phone number can deliver.

Mr. Baker concludes that, far from becoming easier, “the ability to engage a customer through e-mail across a portfolio of communications is becoming more costly and complex.”

With 4 billion e-mail addresses sloshing around in the digital space, there’s no doubt e-mail marketing will continue to be a major force in marketing. Even if half of them are cyber-zombies, digital Potemkin villages or what-have-you.

The challenge is in sorting it all out.

I think the e-mail specialists are going to be at this for a good long time to come.