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.