Experts estimate that the data includes information from 272 million accounts.
Hundreds of millions of user names and passwords from emails and other sites are offered by a hacker to criminals on Internet forums in Russia, a security expert said.
The discovery of 272.3 million stolen accounts includes a majority of users of Mail.ru, the most popular email service in Russia , and a smaller number of users of Google, Yahoo and Microsoft, said Alex Holden, founder and president of security information in Hold Security.
After eliminating duplicates, Holden said, the information package contained almost 57 million accounts of Mail.ru, a large portion of the 64 million active monthly email users that Mail.ru said it had at the end of last year.
Hot Mail accounts were about 40 million, or 15% of the 272 million identified, while 33 million, or 12%, belonged to Microsoft Hotmail users, and 9%, or nearly 24 million, were from Gmail, according to Holden, who added that thousands of German and Chinese email provider accounts were found.
Thousands of other user combinations and stolen passwords seemed to belong to employees of some of the largest US companies in the banking, industrial and retail sectors, Holden added.
" This information is strong, it is floating around in the underworld and this person has shown that he is willing to give it to people who treat him well ," Holden said.
After being informed of the potential theft of email accounts, Mail.ru spokeswoman Madina Tayupova told Reuters: "We are checking if any of the username / password combinations correspond to user mails and if they are still active" .
A spokesperson for Microsoft, meanwhile, said that online identity theft was an unfortunate reality. Yahoo and Google did not respond to requests for statements.