I think it's interesting to compare the way e-mail works to the way the Postal Service works. Let's look at it from the perspective of the Postal Service handling our e-mail.
First off, everyone pays to send and receive e-mail. Even if it's indirectly (an advertisement in your mail or on your mailbox) someone's paying for your mail. Instead of buying postage for each item you send, you pay a lump sum every month - around $2 per month let's say. Each item you send has the same rate - a postcard and a package are the same under your monthly fee. However, you can only send packages up to 10lbs in weight. You can also only store up to 100lbs (that's not to scale) of mail in your mailbox at any time. If you want to send more or keep more mail, you'll have to pay more per month.
When you mail a letter [e-mail], it goes to your local post office. But what then? In the world of e-mail, every post office is basically a separate fiefdom and subject to its own laws. Your post office will try to deliver your mail to where it needs to go, but it may have to go through several other post offices along the way, and all of them have different rules about whether it is allowed to be delivered. Most of it comes down to whether or not they determine your message is unsolicited bulk mail [spam]. If they think your mail is too suspect, or you're sending too many pieces of mail at once, or you sent it from a post office that may send a lot of spam-like letters, or a number of other reasons: they will not accept your mail. You or your post office will have to convince the other post offices that indeed your letter is a real and honest letter and should be accepted.
Companies that advertise via mail in this way are subject to a common problem: post offices refusing their mail. Sometimes it's genuine; a company may be sending what amounts to spam, and any given post office may not want to pass that on since it may be filling up their queues with excessive mail. A lot of the time the mail is genuine but the post office still deemed it to be spam. Either they're just sending too much and seem suspicious, or some of the recipients don't live at those addresses anymore, who knows. Maybe they just don't have the right official papers [SPF records] and the post office wants them to prove they're real. Everybody has their own criteria and they can do what they want in terms of sending your mail through or not.
It's not their fault. The poor post offices are under siege by tons and tons of mail which most of their users don't want. If the post offices weren't so selective, you'd have full mail bags dumped at your mailbox every day and it'd be nearly impossible for you to sort out an important document from the IRS or something like that. They need to verify that the mail coming in is vaguely genuine or it'll just clog up the whole system.
This isn't an exact representation of how the system works, but it's close. The sending and receiving of mail is basically a game of can-i-send-it, should-i-accept-it, with lots of negotiating and back-and-forth just to get some letters through. But how to solve it? Lots of suggestions have been put forth, even to the point of additional requiring fees to receive or send mail. But I think the heart of the problem lies in the unregulated nature of what is now an essential, core method of communication for the people of the world.
Postal mail is regulated, and though that body is woefully out of date and badly managed it does provide the mail in a reliable and fairly timely manner. The system allows for 3rd parties to also function to ensure a delivery of a message. This needs to be taken into account with our e-mail systems. We should have a regulated method of reliably delivering e-mail, but also allow for 3rd parties that can ensure the delivery of a message. A similar system would require each internet user's mail box to be independent of any particular provider so that anyone anywhere in the world can come and drop mail in it, without need to pay for the privilege of receiving said mail and without thought of whether or not my post office will allow me to receive it [spam blocking]. E-mail should become the next necessary component of communication amongst humans, but to do this you need to break down the barrier of cost and technicality.
I think the idea of requiring a small fee to send some mail is time-tested and effective. Would you spend $0.10 to send a letter to an editor telling them how they're a nazi because their news story had a certain slant? And would spam companies be able to deliver such huge mountains of unsolicited bulk mail if each one cost them $0.10? It wouldn't fix the problem because you'd still have people who have it in their budget to spend a gross amount of money on targeted advertising/marketing, but they couldn't afford to simply mail every person in the world for something they're probably not going to buy. This could also help fund a central regulated organized system of vetting mail and ensuring it gets delivered, instead of having all different post offices decide if they'll deliver you your mail or not.