Stored Communications Act and City of Ontario v. Quon
The SCA bars unauthorized access to stored electronic communications, with exceptions for the service provider (often the employer) and an authorized user. Employers may generally review work communications for reasonable, work-related reasons. In City of Ontario v. Quon, the Supreme Court allowed review of an employee's texts to check policy compliance. ECPA does not preempt stricter state laws.
The SCA generally prohibits unauthorized access to electronic communications in storage. Two exceptions may apply to employers: conduct authorized by the provider of the service (often the employer) and by an authorized user. Employers may generally review workers' electronic communications if the reason is reasonable and work-related.
In City of Ontario v. Quon, the Supreme Court allowed an employer to review an employee's text messages to check whether its electronic-usage policy was violated; the employer had provided the pager. Note the employer was a public entity, so the Fourth Amendment (not just ECPA) was implicated.
ECPA generally does not preempt stricter state laws (some states protect email). The private vs public employer distinction is especially important in ECPA cases.