AS 25653 FortressITX went to zero for one day, 15 May, in the May 2012 U.S. SpamRankings.net, but bounded back up to more than 294,000 spam messages a day a week later, placing #6 for the month as a whole.
This was the second time FortressITX made the U.S. top 10. It had been #9 in March, but had dropped out of the April 2012 U.S. rankings. And yes, it's snowshoe spam. That ASN does show a few other problems, also not botnets.