For those curious about the legal justification it is as follows.
When the email system was first created everyone was using a protocol called POP (or POP3 as you might have heard of it), which is still around today. When you check email using this protocol, you would download all your email to your local machine and delete all remote email off the server. This was because email servers were considered more of a cache for your emails than an actual long term storage device like they are now. Later, a protocol called IMAP allowed this exact same functionality and you might be using this protocol right now.
Later, web mail products like Yahoo! Mail popped up offering a website to access your email, then GMail, etc. Now we all use either IMAP, Exchange, or web based mail, which are all tools for keeping the email on the centralized server.
But the law never changed after it was written, and it was written during the POP age. The law was written so long ago that email after six months on a POP server was considered to be "abandoned". At that point, the legal analogy being made was as if you lived in an apartment/house, moved, and six months later they found a bunch of opened mail in the building. That is why the requirement is also that the email must be tagged as "read", because they legally cannot open a letter addressed to you that is closed when the mail is abandoned.
This is so ridiculous.. and very very sad, considering it probably wouldn't occur to most people that the people who make the rules are, by definition, above the rules. If you're in a position to make a rule that everyone else has to follow, under threat of punishment, then you are a ruler. Imagine a medieval King with today's technology. Would he need permission (from himself?) to read his subjects' e-mails? No. Would he want to read everyone's mails? Of course he would, because that would cement his position as a ruler, because by monitoring everyone's comms, he can spot anyone being a threat to him (by speaking the truth) and proceed to make a gruesome example out of the pesky thought-criminal. People think that a pack of wolves will somehow limit their own sheep-eating activities, by writing some rules on a piece of magic paper (that magically prevents them from eating sheep).It's no longer a surprise that the government is reading your emails. What you might not know is that it can readily read most of your email without a warrant.
So, if I were to whip up some script that went through every single email account I had and mark everything older than 6 months as unread on a daily basis, I would be immune to this type of search? On the other side; This is just another example reminding us that technology is advancing faster than we can regulate it.
It has to be unread and 6 months old, so if you just select all of your email and mark it as unread that is older than say 4 months, and go back every month and do the same, then yes technically you should be immune. There are also sneak and peek warrants, and IIRC they do not have to notify you if they do that for the purposes of "national security". Plus, the only safeguard here is that they "promise" to obey by the rules, any one corrupt officer can just look at your email without warrant and nobody would really know. I think the best protection is just to take your email off the server, and take your data out of the cloud. Anything on the internet is basically fair game under some law or another, or 5, or 20. If you have to use the cloud still, especially for email, just remove all email every 5 or so months and archive it.
It's difficult for many people to run their own mail servers (really fun convincing all the other servers in the world that you aren't a spammer), and that only works if all parties in the conversation do it. As for mitigating it, I wouldn't expect deleting my mail on someone else's server to make it unavailable to anyone but me. End-to-end encryption is the answer. Even a simple trust-on-first-use scheme makes the barriers to eavesdropping much higher (i.e. existent). It still has network effect problems though - most people use Android email clients that don't support encryption (presumably iPeople are in a worse boat, but that's their choice), most people use webmail for some reason, etc. People at large seem to be achieving the first glimmers of lucidity on the importance of privacy in The Information Age, so hopefully that can be overcome. The only downside once more people are using it is that it destroys the revenue model of all free email providers (i.e. reading your mail).