IRS claims it can read your e-mail without a warrant

Indeed. Making something that could be made impossible (or extremely difficult) "only" illegal will tend to make people complacent. If the expectation is that bad people won't do something because it's against the law, good people may fail to ensure that they can't. And then bad people do it anyway, leading to outrage, panic and harmful reactionary legislation. Not to mention bad people getting away with doing bad things, which could all have been prevented with sound engineering.

This isn't to say that every problem can be solved with a technical solution -- but when the technical solution is extremely effective, a legal solution is surplus to requirements.

This is true, and a great point. I often open my neighbors' postal mail using this same excuse. Sometimes you can even read the letters and notices right through the paper envelope!

So let's take that argument to its extreme: you are standing in an open field going to the bathroom. Do you still think you have an reasonable expectation of privacy?

The problem with your argument is that it is based on the idea that if people want privacy, they are entitled to it even if they do nothing to protect that privacy. There is nothing wrong with expecting people to be a bit proactive when it comes to their privacy -- closing doors, drawing their curtains, encrypting their email. People should not just shrug about someone filming them on the toilet, they should do something to prevent it (and if a person started to drill holes in the door, sure, prosecute them -- for destroying someone's property).

"If someone were going into a restroom taking pictures, people would feel rightfully violated"

No, if someone were going into a restroom taking pictures of other people going to the bathroom in a closed stall they would rightly feel violated. Any weaker standard is basically saying that nobody can take pictures in public, because they might accidentally catch a person peeing on a wall and thus violate their privacy.

"The ease with which it can be done is irrelevant to the legality of it."

No, it is very relevant to the legality, because when we make easy and popular things illegal we worsen an already out-of-control criminal justice system. We do not want to start arresting people just because they are using their cameras, even if their cameras are mounted on their heads, even if they wear their head-mounted cameras into bathrooms. If cameras are everywhere and used by everyone, the answer is to build bathroom stalls that go from the floor to the ceiling, not to open the door to waves of prosecution (which will almost certainly be used selectively against "undesirable" people).

The appropriate place to draw the legal line with privacy is with people taking proactive and reasonable measures to be private. Putting letters in envelopes, closing curtains, closing doors, and yes, encrypting emails. You have no expectation of privacy in public parks, nor if your door is left open, nor if your curtains are left open, so why should you expect privacy when you send unencrypted email?

With that said, it's actually a Federal crime to remove mail from someone else's mailbox to obstruct or pry into their business (even for postcards), which would seem to support your overall point.

http://www.law.cornell.edu/uscode/text/18/1702

Because I don't want technology to rule us, I want us, humans, to rule technology. We say a lot of times that "technology is a tool". If we have to adopt ourselves and our society to it, instead of adapting it to our preferences, goals and morals, then it's not a tool, it's a ruler.

In this case, cryptography might be a solution. But it's not a perfect solution for me. For one, it's not widespread and it's confusing for most people to integrate to their mailing habits. Second it breaks lots of workflows and conveniences (e.g full text search of emails).

Second, I don't want the government, Google, or anybody else to have it be legal to look into my email if they can break the cryptography or find the key. I want it to be illegal even at that case.

Third, while cryptography might be a case were technology can solve this problem (privacy) there are other issues just piling technology cannot be used to solve them -- where legislation is needed.