This is the guest post written by Steve Chou.
Email spam is a huge problem among ISPs. Because email has no gatekeeper, anyone can send an email to whoever they want under any name and impersonate other senders.
What’s worse, spam can be sent out in massive volume by bots and networks of infected computers.
If you take a quick look at your spam folder, you’ll probably see fake or spoofed emails from well known companies and even your friends! For example, I receive at least 2 or 3 fake emails impersonating Paypal every day.
How does your ISP know the difference between a real and a fake email? One way to tell whether an email is genuine or not is with DKIM.