(It's actually worse than this quote suggests, as SpamArrest are spamming lots of people who have not only never sent email to an @spamarrest.com email address but also never sent mail to any customer of SpamArrest. Not that it would be OK or 'not spam' even if they were only sending mail to people who had mailed their customers).
SpamArrest is a fairly broken challenge-response spam-filtering system.
As part of their spam filtering they harvest the email address of everyone who sends email to one of their users, or who sends email to a mailing list one of their users is subscribed to.
They've recently decided to spam all of those harvested addresses. One example can be found here but I've seen dozens of other reports.
The spam sent violates California state law, and probably some others.
When asked about this, Daryn Nakhuda believes their unashamed spam is a perfectly legitimate marketing approach:
The number of critical points where this statement diverges wildly from reality is concerning.
To avoid spam from spamarrest in the future I would strongly suggest everyone who doesn't want spam from spamarrest
It isn't adequate to just remove any @spamarrest.com email address from a mailing list as any email address can be subscribed to a mailing list, then forwarded to spamarrest to cause challenges and harvest the mailing list participants email addresses.
SpamArrest are unavailable for comment by 'phone (their listed contact number on their website leads to a 'full' voicemail box and their registered contact information appears to be false - the voice number goes to a fax line). Their provider, Internap, are deeply confused as to whether they permit their customers to send spam or not.
Some relevant links: