Google has confirmed that its Gmail service has been targeted by an "industry-wide phishing scheme".
A spokesperson for the internet giant told BBC News that the affected accounts had had their passwords reset.
"We will continue to force password resets on additional accounts when we become aware of them," they added.
The scam, in which the passwords to over 30,000 email accounts were reportedly posted to a website, was initially only thought to have affected Hotmail. However, the BBC has seen lists said to also include Yahoo and AOL addresses.
All the account details have now been removed from the site.
Phishing scams involve using fake websites, such as email or online banking services, to trick people into logging in and disclosing personal details.