I would keep it as small as possible, but several things do need to be spelled out. I use the phrase communication systems to include all forms of communication .. email, IM, telephone, intranet, blog, collaboration spaces, etc.
1- The corporate communication systems are not private. Any communication through a company network or service, whether personal or work-related, is the property of the company. The company can monitor, review, search, and re-publish any communication made using it's network or service, at it's sole discretion.
2- Disrespect, abuse, or poor treatment of other people, whether they are company associates or outsiders, is unacceptable and not allowed. This includes, but is not limited to, personal insults, abusive language, profanity, explicit or implicit sexual references, and reference to any "protected" class or status. This policy applies whether the target of said treatment is a party to the communication or not.
3- Copyright and authorship are to be respected. Any material written by another that is used in any form of ecomm must be referenced as such. If authorship is not known, that must be indicated.
4- Extreme care must be taken when dealing with confidential or proprietary information.
5- Written communication should follow the company style guide and other published guidelines.
6- All employees are expected to exercise sound judgment when using company communication systems.