MailBlocks

Mailblocks offered free challenge-response spam filtering web email service and an IMAP interface as a revenue service.^[2]

Mailblocks did not invent challenge response to block spam. Instead, they purchased the rights to two patents related to challenge response: patents US6199102 and US6112227. Mailblocks then proceeded to patent troll several other companies before releasing any product of their own. Companies sued included Spam Arrest, DigiPortal, MailFrontier, and Earthlink.^[3] A challenge response was a well known technique for fighting personal spam and mailing list spam for years before the two patents were applied for. For example, David Skoll described it in detail in a post to a public forum on November 15, 1996.