More than 30,000 user names and passwords were posted on a site used by technology experts last week.
The data has since been removed.
The companies say it was part of a scam to get users to give away their personal information to hackers.
The e-mail providers say security on their servers was not compromised.
The companies are urging anyone with a Hotmail, MSN, G-mail or AOL e-mail to change their password.