filtering systems which let through only those messages that have identified themselves as "ham". Reporting spam Tracking down a spammer's ISP and reporting the offense can lead to the spammer's service being terminated and criminal prosecution. Unfortunately, it can be difficult to track down the spammer, and while there are some online tools such as SpamCop and Network Abuse Clearinghouse to assist, they are not always accurate. Historically, reporting spam in this way has not played a large part in abating spam, since the spammers simply move their operation to another URL, ISP or network of IP addresses. In many countries consumers may also report unwanted and deceptive commercial email to the authorities, e.g. in the US to the US Federal Trade Commission (FTC), or similar agencies in other countries. Automated techniques for email administrators There are now a large number of applications, appliances, services, and software systems that email administrators can use to reduce the load of spam on their systems and mailboxes. In general these attempt to reject (or "block"), the majority of spam email outright at the SMTP connection stage. If they do accept a message, they will typically then analyze the content further – and may decide to "quarantine" any categorised as spam. Authentication A number of systems have been developed that allow domain name owners to identify email as authorized. Many of these systems use the DNS to list sites authorized to send email on their behalf. After many other proposals, SPF, DKIM and DMARC are all now widely supported with growing adoption. While not directly attacking spam, these systems make it much harder to spoof addresses, a common technique of spammers - but also used in phishing, and other types of fraud via email. Challenge/response systems A method which may be used by internet service providers, by specialized services or enterprises to combat spam is to require unknown senders to pass various tests before their messages are delivered. These strategies are termed "challenge/response systems". Checksum-based filtering Checksum-based filter exploits the fact that the messages are sent in bulk, that is that they will be identical with small variations. Checksum-based filters strip out everything that might vary between messages, reduce what remains to a checksum, and look that checksum up in a database such as the Distributed Checksum Clearinghouse which collects the checksums of messages that email recipients consider to be spam (some people have