Both tech companies have reported a huge amount of spam emails that use their brand names to convince receipents they've won a lottery. The scammers then instruct victims to pay fees in order to collect their winnings, a scam known as advance fee fraud.
The African Development Bank and Western Union, whose brands are also used in the scams have also joined Microsoft and Yahoo.
People who have been scammed should report the crime to their local police authority. Then, the four companies would like people to send them the police reports by email or post. Their internal investigators will look for patterns and other data and pass it along to police. The international law enforcement agency Interpol will also work with countries to help coordinate the program.
Indicators show that a relatively small number of gangs are responsible for administering the scams, said Roger Halbheer, Microsoft's chief security advisor for Europe, the Middle East and Africa. The scams are cheap to run, since they often send email through free services. They're also quite lucrative, he said.
A recent Microsoft survey of 4,930 internet users in the UK, Germany, France, Italy, Spain, the Netherlands and Denmark revealed that 113 people had lost from €100 (£79) up to €7,000 (£5,560) to an internet scammer in the past 12 months. Put another way, one in 44 people had been scammed.
Someone running a direct mail campaign would probably be 'manager of the year if you had that kind of return', Halbheer said.
People are encouraged to send their police report to the following addresses:
Alternatively, scam victims can also visit a dedicated Yahoo web page.