NEW YORK (CNN) - Nearly one in three U.S. companies has terminated an employee for violating e-mail policy in the past year, a survey released Monday said.

The survey of 294 U.S. companies with at least 1,000 employees found that, in many cases, employees who sent e-mails containing confidential information about the organization or e-mails that were considered obscene and offensive were disciplined or terminated.

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Although the survey suggests that 80 percent of U.S. companies have a written policy covering e-mails, violations remain common, said the study, which was carried out by Forrester Consulting and paid for by Proofpoint, a Cupertino, Calif.-based company that makes software to help companies reduce e-mail risks.

Proofpoint reported that employers are ever more concerned that outgoing e-mail could pose a risk to the company. As a result, many companies are hiring staff to read and analyze outbound e-mail. According to the study, 44 percent of major U.S. companies have hired employees to do so.

Blogs and Instant Messaging are also a concern to many employers. Seven percent reported that an employee was terminated for such a violation while 17 percent reported that an employee has been disciplined.

Federal prosecutors have used e-mails to build cases against a number of high-profile executives.

The Department of Justice seized Enron's e-mail records, and used them as evidence in their successful prosecution of company executives Ken Lay and Jeffrey Skilling.

Prosecutors charged investment banker Frank Quattrone with obstruction of justice, alleging that he asked colleagues at Credit Suisse First Boston, during a legal investigation, to delete e-mails (see correction).

The survey has a margin of error of plus-or-minus 4.76 points.

-- by CNN's Maria Gavrilovic

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Correction: An earlier version of this story implied that Frank Quattrone had asked for e-mails to be deleted. This version clarifies that as the allegation brought by federal prosecutors. CNNMoney.com regrets the error.

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