"This is your bank/IRS. There's a problem with you account/taxes. If you don't respond by date X we'll close/fine your account/you. This is the only notice you will get."
They sent one reminder, which I also missed. By happiest of coincidences, I logged into their ticketing system to look up a Linux command a support rep had taught me once, and saw the tickets.
(The AUP violation was a server getting hacked, probably via a WordPress blog, and being used to attack SSH on other servers. I nearly had a heart attack, until I realized "Hey wait, that is not actually an IP I own." Turns out they had the wrong account. "Sorry about the mistake!" I don't think that would have cut it, honestly, if they had actually gone through on the deactivation threat.)
Still, I agree, important messages through email should always be replied to to confirm that you've got them.