Habeas Warrant Mark
Habeas Sender Warranted Email was an unconventional attempt to fight email spam. It is apparently defunct.
It used a so-called Habeas Warrant Mark, which was a set of email headers containing a copyrighted haiku, and various trademarks. Non-spam emails were authorized to include the Warrant Mark in their headers, but any spammers who distributed the Warrant Mark would be added to a blacklist. Spam filtering software could utilize the blacklist, and/or judge an email containing the Warrant Mark as unlikely to be spam. Legal challenges to the blacklist would be risky, because Habeas could countersue the spammer for intellectual property infringement.
The first few lines of the Warrant Mark contain the haiku. The remaining lines are reproduced below.
... X-Habeas-SWE-5: Sender Warranted Email (SWE) (tm). The sender of this X-Habeas-SWE-6: email in exchange for a license for this Habeas X-Habeas-SWE-7: warrant mark warrants that this is a Habeas Compliant X-Habeas-SWE-8: Message (HCM) and not spam. Please report use of this X-Habeas-SWE-9: mark in spam to <http://www.habeas.com/report/>.
- How Sender Warranted Email works (from archive.org)