The board's 3-2 decision Friday involved e-mails sent in 2000 by the president of the Newspaper Guild unit at the Eugene Register-Guard in Oregon, The New York Times reported. The board majority said that employers who ban employees from sending solicitations for any outside groups can ban union e-mails as well.

"An employer has a 'basic property right' to regulate and restrict employee use of company property," the ruling said. "The respondent's communications system, including its e-mail system, is the respondent's property."

The two members in the minority questioned the property interest in e-mail systems and said the majority did not balance it against employee rights.

The decision outraged many labor leaders.

"Anyone with e-mail knows that this is how employees communicate with each other in today's workplace," said Jonathan Hiatt, the AFL-CIO's general counsel.