Not entirely. The National Labor Relations Board decided on December 11, 2014 that it was not lawful for an employer to prohibit an employee from using his or her work email account for certain types of protected activity, such as discussing union organizing and urging other employees to get together to protest working conditions. The general counsel of the NLRB argued that email is the present-day “water cooler” of the workplace, and that employees should be able to use this medium of communication to fulfill the rights that they have under the National Labor Relations Act to work together for a better deal.
However, an employer may lawfully discipline an employee for non-protected, personal use of a work email account. For example, unlawful or inappropriate communications would not be covered by this ruling- you can’t sexually harass a coworker or set up an illegal drug buy and then complain that your rights were violated. Even strictly personal use of a work email account can fairly be subject to discipline- arranging your vacation or emailing your friends about fantasy football can still get you in hot water.