Friction
Stamps (remember those?) make direct mail work. Because it costs money to send a piece of junk mail, you'll think two or three times before you mail something to a million people.
Email, of course, is free.
Except it's not. The friction that slows down sending email to everyone all the time is the cost of all the people you'll lose. You might lose them because they unsubscribe, or more likely, you'll train them to ignore you. Worse still, you might just make them annoyed enough to badmouth you.
Drugstore.com made two mistakes with their relationship with me. First, they bought the lie that opt out is a productive strategy. They unilaterally decided that I'd be delighted to get regular emails from them, merely because I bought some shaving cream.
The second mistake? They didn't bother to be selective about what they sent.
I've never purchased diapers online, since my diaper purchases predate online diaper shopping. And my hope is that I won't be buying Depends for another fifty years or so. Drugstore.com should know this. And yet, because it's apparently free to email me, some lame brand manager says, "sure, do it!"
Except then I unsubscribe and an asset that is worth ten or a hundred or a thousand dollars disappears, probably forever.
Find friction and embrace it, don't ignore it.