Cheap Tricks: The Low Cost of Internet Harassment — ProPublica

Later I learned that the type of attack aimed at my colleagues and me is often called “email bombing” or “subscription bombing.” It’s clever jujitsu that turns one of the hallmarks of spam prevention — the confirmation email — into a spam generator. It works like this: The attacker uses an automated program to scan the web for any sign-up form that asks for an email address, from a newsletter subscription to an account registration. It then inserts the target’s email address into each of the forms, flooding the victim with confirmation emails.