However unlikely, the "imminent danger" clause has standing before the law. So in the case of the proverbial "ticking bomb" it would be justified. I have a little trouble believing snail-mail would ever be involved in a "ticking bomb" speed scenario -- as are most of you.

The other reason -- continuing threat -- seems a valid reason to search suspect mail, but I simply do not see the need to do so without a warrant. I cannot imagine a federal judge refusing to sign promptly such a warrant if there is any plausible reason for suspicion....and we do have federal judges cleared for security sensitive rulings.