U.S. securities regulators are warning investors of a notable jump in email spam touting pump-and-dump schemes.

The U.S. Securities and Exchange Commission (SEC) and the Financial Industry Regulatory Authority (FINRA) issued a warning Wednesday indicating that the latest McAfee Threats Report signals a steep rise in spam email linked to pump-and-dump stock schemes designed to trick unsuspecting investors.

It says these scammers frequently claim to have “inside” information about an impending development. Others may say they use an “infallible” system that uses a combination of economic and stock market data to pick stocks. “These scams are the inbox equivalent of a boiler room sales operation, hounding investors with potentially false information about a company,” the regulators note.

They stress that these scams could also be touted on social media, such as Facebook and Twitter, as well as on bulletin boards and chat room pages.

“Investors should always be wary of unsolicited investment offers in the form of an email from a stranger,” said Lori Schock, director of the SEC’s Office of Investor Education and Advocacy. “The best response to investment spam is to hit delete.”

“Spam email is the bait used to lure people into making bad investment decisions. No one should ever make an investment based on the advice of an unsolicited email,” added Cameron Funkhouser, executive vice president of FINRA’s Office of Fraud Detection and Market Intelligence.