If you received a threatening or harassing email reply to your ad, you can report it to Craigslist by opening the email and clicking on the Please flag unwanted messages link at the bottom. This link, however, is only present if you opted to use Craigslist's email relay service when you posted your ad. Emails that Craigslist sends through its relay come from addresses ending with @relay.craigslist.org. If you chose not to use the relay service, you can report the harassing or threatening email directly to the sender's mail provider.
Personal Information Posted on Craigslist
If someone posted some of your personal information -- such as your name, address or phone number -- on Craigslist without your consent, you can file a complaint by clicking on Personal Harassment on the Craigslist Contact Form page. After selecting the type of personal information that someone posted on Craigslist, the website then redirects you to a contact form that allows you to provide details about the abusive posting.