Legal Precedent
An interesting precedent is Rosenfeld v. Zerneck, a New York State court decision on the validity of an email signature.
Court Decision in Rosenfeld v. Zerneck
National Notary Organization Commentary on Rosenfeld v. Zerneck
Legal Context
Currently the definitive guide is U.S. Department of Justice LEGAL CONSIDERATIONS IN DESIGNING AND IMPLEMENTING ELECTRONIC PROCESSES: A GUIDE FOR FEDERAL AGENCIES, from which I quote at length below:
...