Yes, Internet and e-mail use can be covered in an Acceptable Use Policy. Both Internet usage and e-mail services are company provided resources and the organization can state what usage is acceptable and what is not acceptable, as well as the enforcements in place for violations of the policy.
Internet usage could spell out what websites are permitted during work hours, as well as what websites are never permitted at any hours. Email related policy statements could include that the organization owns all the email and that it should be strictly used for official purposes.
Also, the organization could explicitly spell out that as the owners and providers of the resources they have the right to monitor all traffic and information.