WALTHAM, Mass.--(BUSINESS WIRE)--#cloud--GreatHorn, provider of the only email security platform that protects organizations before, during, and after an email attack, today announced it has closed a $13 million funding round co-led by RRE Ventures and .406 Ventures, with participation from existing investors including Techstars Ventures, V1.VC and Uncork Capital. On the heels of its third consecutive year of triple-digit revenue growth and rapid expansion into the enterprise market, GreatHorn plans to.