Other targeted items included television sets, home-networking routers, computers and multimedia centres.
A California-based security group has discovered the first fridge to send out spam.
Proofpoint said the fridge was one of more than 100,000 devices such as smart TV sets, computers and multimedia centres that hackers broke into.
They then used the devices to send more than 750,000 malicious emails to businesses and people around the world.
The attack is thought to have taken place between 23 December 2013 and 6 January this year, the security firm said in a statement.
About 75% of the emails were sent by regular computers, while the rest were sent by hacked home appliances.
David Knight, General Manager of Proofpoint’s Information Security, told the BBC: "The results spoke for themselves when the addresses responded with explicit identification, including well-known, often graphically branded interfaces, file structures, and content.
"Many of these devices are poorly protected at best and consumers have virtually no way to detect or fix infections when they do occur.
"Enterprises may find distributed attacks increasing as more and more of these devices come online and attackers find additional ways to exploit them."