That's a theory I also subscribe to, but not with Hushmail, more likely the mixmaster network is somewhat compromised. Serious Operatives conducting serious dealings don't use services like Hush, it's fundamentally insecure by the very fact that they have possession of one's private PGP key.
The best way to hide recipent/sender data and message content is to use programs like Jack B Nymble to post through the mixmaster network, and use self encrypted messages/attachments.That's not 100% secure ,but damn near.
If one was to alternate the initial mailgate for entry into the network, on the assumption that the network has a compromised relay or two, and connect from alternating anonymous points like Kinkos, libraries, Internet cafes, etc. one would be virtually untraceable.I say virtually, cause Death and Taxes are the only 100% sure things I know of. Even if the NSA is running one or many of the mixmaster relays, they don't give a damn about Anabolics and they don't readily share info they've picked up with anyone.
It's my opinion that the NSA can't crack properly done encryption,and I do not mean RSA's PGP, and the Justice Department certainly can't.The sender/receipent data may be catchable, but the content can be unreadable by anyone if one really knows encryption.
This is a complex subject, beyond the scope of a discussion here. Most people would do well to incoroporate their own encryption into their use of services like Hush or other emailers.