A test by the Information Security Research Team (INSERT) has uncovered a flaw in Google’s Gmail service that allows spammers to send thousands of junk emails that pass right through most spam filters.

Email providers typically blacklist the IP addresses of known spammers, but reliable email clients, like Gmail, pass through filters freely. INSERT was able to send a bulk email to 4,000 people in a six-hour time frame using one Gmail account, according to BetaNews. Google usually caps bulk emails at 500 addresses.

Another test found that emails sent from blacklisted IPs through Gmail were much less likely to be blocked than emails sent from Yahoo and Hotmail accounts.

INSERT has notified Google of the flaws and is withholding further details of the study until Google has a chance to fix the security breaches.

The spam loophole is the latest in what seems to be a trail of controversy following Google these days. In March, there was debate over the potential for the search giant to steer users toward Google-owned properties, like YouTube. Last month, webmasters were abuzz about changes to Google's toolbar PageRank, with many saying they saw dramatic shifts in their scores.