Why does email get lost?

First, I need to clarify an assumption made in the question:

Wrong.

In the beginning, this may well have been the case. In the early days of email and the internet mail volume was low enough, and mail abuse hadn't yet skyrocketted, so it was feasible for mail servers to do exactly that: delivery, or report why they couldn't.

Today that's simply not possible. Most mail servers will ignore mail for many, many reasons. And by ignore I mean they'll receive it, and then discard it without telling anyone. We'll see why they often take this approach in a moment.

But as a hint of things to come, you'll be seeing the word "spam" a lot.

So, on to some possible reasons...

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Invalid Recipient

If you make a typo when you enter the email address you're attempting to send to you're sending to an invalid recipient. Many mail servers are now configured not to respond to those with a bounce message.

The problem is that one technique used by many spammers is to blast email messages to thousands of email addresses whether they exist or not. So you might see a series of emails sent to "len@", "leo@", "leon@" all on some domain. If just one of those is valid, the spammer has reached closer to a target. The others? The spammer doesn't care. It's easy and cost effective to send a thousand email messages just to reach that one valid email address.

So why not bounce? Two reasons: the receiving mail server would be spending much, if not most of its time doing nothing more than sending bounce messages as all this spam comes in. In addition, the bounce messages themselves take up time and bandwidth on the internet. Given that 80 to 90 percent of all email is spam these days, the impact of all those invalid email addresses generating even more email is horrific. Not to mention that since many of the "From:" addresses are spoofed and invalid, the bounce messages themselves might only generate more bounce messages in return.

I've seen it in action, and the impact on server load and bandwidth of changing from bounces to just ignoring invalid email recipients is significant.

Invalid Sender

This can cause problems in a couple of different ways.

Sometimes ISPs and spam filters will consider your email a little more "spammy" if it's sent from a server that has a different domain than the email address on the From: line. Combined with other spam recognition techniques, that can sometimes push you over the line and cause you to be considered spam. Personally I don't see this very often any more because it's so very common to send email from one domain, say a personally purchased one, using your ISP's mail server.

Of course if you misconfigure your email program and type your own email address wrong, that can cause problems. That gets placed into the "From:" field of outgoing email, and not only can recipients not just reply to you, but bounce messages, if they are generated, will get sent to the wrong address.

The Content Looks Like Spam

If the content of your message contains words and phrases that are considered to be "spammy", then your message might be mistaken as spam. Sometimes spam filters aren't all that smart either. I've had one filter out messages because it had the word specialist in it. Specialist, of course contains a drug name within it that we often see in spam: specialist. Spam filters should be smarter but many are not.

But the bottom line is that the content of your email message will be analyzed by spam filters along the way, and if there are too many similarities to known spam, it may well be discarded.

The Sender Looks Like a Spammer

One of the other ways that spam is identified is by the sender's email address. If someone sends a large amount of spam from a single email address, spam filters may take that address and decide it's a "known spammer", making future email from that same sender much more likely to be marked as spam.

Spam filters can maintain these types of known spammer lists locally, or there are often publicly available blacklists that once you're on are nearly impossible to get off.

Sender blacklisting is falling out of favor, though, because "From spoofing" is so common place now. "From spoofing" is sending spam with someone else's (valid) email address in the "From:" field, even though they had nothing to do with the spam.

The Sender's Provider Looks Like a Spammer

More common than email address blacklisting is email server blacklisting. In this case a large volume of spam is traced as being sent from a single mail server. That server might then be blacklisted as being a known source of spam.

The problem is that you might be using that same sever, and using it legitimately. For example, if a spammer is using your ISPs mail server to send spam, then that server could get blacklisted. Since might use that same server, then your mail would look like it came from a "known spammer".

These blacklists are also difficult to get off of. In many ways, they're also slowly falling out of favor, due to the rise of zombies and botnet which send spam from thousands if not millions of machines.

The Bounce Looks Too Much Like Spam

It's rare, but possible that you might get sent a bounce message, and that a spam filter along the way might consider it to be too much like spam, and not deliver it.

Shouldn't Spam Always be Delivered?

The fact that you have a spam folder unfortunately doesn't mean that all messages considered to be spam will be delivered there. Depending on many, many factors, large ISPs often block what they consider to be spam long before it ever stands a chance of reaching your account, much less your spam folder.

Ultimately spam can be blamed for most undelivered email. Either directly: email itself is mistakenly considered to be spam, or indirectly as the processes and procedures that are put into place to prevent spam simply put up too high a barrier for some email to be delivered.