You need more than e-mail
I’ve seen it again and again — firms that had critical projects go off the rails, simply because they did not insist on having signed contracts from their vendors and other key contributors. Important aspect of project management consulting is to minimize risk — and you introduce a lot of unnecessary risk by relying on verbal agreements or e-mails alone as the basis of your working relationships. Unless that individual is a paid employee of your corporation, you need to have a signed contract in place so that both parties clearly understand what the project deliverables are, associated time frames and costs/compensation to be given.