Pump-and-dump share spam, which accounted for half of all unsolicited email in February, has plummeted to just five per cent this month, new research has revealed.

"At the beginning of the year, pump-and-dump spam was very popular, peaking at 50 per cent of all spam in February," said Bradley Anstis, director of product management at email security firm Marshal.

Pump-and-dump spam levels have fluctuated over the past quarter, but averaged around 30 per cent.

In the last four weeks of the quarter, however, the Marshal Threat Research and Content Engineering team observed a "rapid decline" in the volume of stock spam to the lowest point in 10 months.

Pump-and-dump spam is a form of financial fraud that involves artificially inflating the price of a stock through untrue or exaggerated promotions.

Once inflated, the spammers sell their stocks to make a profit which usually leads to the stock price crashing, leaving real investors with major losses.

The US Securities and Exchange Commission (SEC) suspended trading of more than 30 companies targeted by pump-and-dump spam earlier this year, to which the decline can be partly attributed.

"Whether the decline is due to the SEC's recent action, overuse by spammers or increased use of advanced spam filtering solutions is moot. Evidently stock spam is significantly less effective in generating profits for spammers," said Anstis.

Pump-and-dump spam is far more risky for spammers compared to other types of spam.

Unlike spam touting pornography or pharmaceutical products, where the spammer is simply trying to sell a product, stock spam requires that the spammers invest some of their own money first.