Lots of the comments here are missing the point. In my experience at enormous companies (that have had e-mail policies, in varying degrees, like this one), they have zero to do with productivity and have everything to do with the various laws, rules, and regulations surrounding data retention of e-mail.
To put this in the best possible light, consider a rogue manager of a publicly traded company, who is discovered by the company and subsequently fired. If he does something bad with a legally-mandated paper trail, we're talking about discoverable material that would show up in any serious lawsuit about an unrelated matter. If we're talking about a crazy internal system that deliberately never logged anything in the first place, the company is open to much less legal liability.
To put this in the best possible light, consider a rogue manager of a publicly traded company, who is discovered by the company and subsequently fired. If he does something bad with a legally-mandated paper trail, we're talking about discoverable material that would show up in any serious lawsuit about an unrelated matter. If we're talking about a crazy internal system that deliberately never logged anything in the first place, the company is open to much less legal liability.