For anyone who wants more details, take a look at Wikipedia's overviews of the relevant laws:

https://secure.wikimedia.org/wikipedia/en/wiki/Stored_Commun...

https://secure.wikimedia.org/wikipedia/en/wiki/Electronic_Co...

Yes, one of the things that matters is how long the email has been stored. The law appears to think that we have less of an interest in the privacy of old emails than we do in new ones for some reason.