Attention is a limited resource

Marketing is all about grabbing attention. You can’t run a successful marketing program without first grabbing attention. But attention is a limited resource. There are only so many things a person can remember, focus on or interact with at any one time.
In many marketing channels there is an outside limit on the amount of attention a marketer can grab. There are only so many minutes available for marketing in a TV or radio hour and they cost real dollars. There’s only so much page space available for press. Billboards cost real money and you can’t just put a billboard up anywhere. With email marketing, there are no such costs and thus a recipient can be trivially and easily overwhelmed by marketers trying to grab their attention.
Whether its unsolicited email or just sending overly frequent solicited email, an overly full mailbox overwhelms the recipient. When this happens, they’ll start blocking mail, or hitting “this is spam” or just abandoning that email address. Faced with an overflowing inbox recipients may take drastic action in order to focus on the stuff that is really important to them.
This is a reality that many marketers don’t get. They think that they can assume that if a person purchases from their company that person wants communication from that company.

Patricia Faley of the Direct Marketing Association counters that businesses have […] the right to contact consumers without first obtaining their permission.
“We call it the ‘one bite at the apple’ rule,” she says. “Give me one chance to show you what I have to offer you, and if you don’t like it, then I won’t contact you again.”

The problem is the sheer number of companies who want to contact each consumer. Even if you limit that to companies the consumers purchase from it’s still an untenable number of contacts. Looking at my pantry right now, there are probably over 100 different brands I’ve purchased. Really, I don’t want every one of them to email me.
Increasing the amount of email sent,  beyond what the users want and outside of their control, weakens the email channel for everyone. Users get so much mail, that they don’t care about any of it. It’s just more noise in their inbox, distracting them from things they want to give their attention to.
Too much clutter in the inbox leads to user dissatisfaction and complaints to the mailbox providers. Those complaints lead the ISPs to want to improve the inbox experience for their users. One way to do that is to filter mail for the user, so that the user only gets mail they really want. ISPs call it engagement, but it’s really just describing how much attention users are giving to that marketer.
Marketers want every bit of attention they can. This isn’t a bad thing, but history shows that when provided with a channel with no price or external constraints that marketers swamp the channel and users revolt. Two obvious parallels to email in terms of cost and control are outbound phone and outbound faxing. Without any sort of control marketers increased frequency to the point where they ruined the channels and no one (or very few) can use those as marketing channels. Marketers tried to get users attention so forcefully that the users revolted.
Is this really where email is going?