We were a Linux System Administration consultancy. The product wasn't the e-mails, but nearly everything we did for our clients was designed/planned, scheduled, organized, and documented in e-mail. Yes, sometimes we would work on things with the client on the phone, those were usually followed up with an e-mail about what was done.

These e-mails were copied to our internal mailing lists so that they could be peer reviewed and someone else could be cross-trained on it in case the primary wasn't available. Also, every task we did had a one sentence description written up that would be shared with the team, again as a kind of peer review.