Report to the ISP
Contact your ISP
If your ISP is aware of a spammer, they may be able to block that spammer from their network. Your ISP generally will not be able to take any direct action against the spammer.
The address to send your complaint to is usually email@example.com. For example, if you are a customer of Juno.com, you should send the complaint to firstname.lastname@example.org. You can lookup the official address for most domains at Abuse.net.
Forwarding your spam to your ISP lets them know about the spam problem on their system and helps them stop it. Include a copy of the spam, along with the full e-mail header, and at the top of the message, state that you are complaining about being spammed.
Most ISPs will reply to your complaint with a generic form letter letting you know they received your complaint. You will usually not receive any other notice from the ISP.
Contact the Sender's ISP
Be aware that the "From:" field of an e-mail message is very easily forged. To know who to complain to you need to read the header information of the e-mail message.
Most ISPs want to remove spammers from their system. The address to send your complaint to is usually email@example.com. So if the spammer is a customer of HotMail.com, you should send the complaint to firstname.lastname@example.org. You can lookup the official address for most domains at Abuse.net.
Include a copy of the message and header information and state that you are complaining about spam.
There are tools available that help you report spam to your ISP. Learn more about these and other tools.