E-mail is dead—or at least that’s what Silicon Valley is banking on. At the end of April, the startup Slack announced it had raised $43 million in funding and said it was valued at about $220 million. Slack has built a type of anti-e-mail messaging system meant to let workers collaborate in a more fluid way. Venture capital firms who joined the gaudy funding round included Social+Capital Partnership, Accel Partners, and Andreessen Horowitz. (Bloomberg LP, which owns Bloomberg Businessweek, is an investor in Andreessen Horowitz.) They clearly think the company is up to something big.