We allow users to sign up for a free trial for our product, you have to put in your name & email address. After the trial expires, we send an email that says "Hey So-and-so, your trial ran out, click here to give us money, etc." Some enterprising spammers filled in the name field with spam URLs and the email field with victims' email addresses, in order to spam them. So the victim would get "Hey hxxp://buyfreerolex.com/, your trial ran out..." spam emails, from our email server. Obviously we've fixed it since, but it's absolutely wild the length spammers will go to.