approach is simply creating an imitation MTA that gives the appearance of being an open mail relay, or an imitation TCP/IP proxy server that gives the appearance of being an open proxy. Spammers who probe systems for open relays and proxies will find such a host and attempt to send mail through it, wasting their time and resources, and potentially, revealing information about themselves and the origin of the spam they are sending to the entity that operates the honeypot. Such a system may simply discard the spam attempts, submit them to DNSBLs, or store them for analysis by the entity operating the honeypot that may enable identification of the spammer for blocking. Hybrid filtering SpamAssassin, Policyd-weight and others use some or all of the various tests for spam, and assign a numerical score to each test. Each message is scanned for these patterns, and the applicable scores tallied up. If the total is above a fixed value, the message is rejected or flagged as spam. By ensuring that no single spam test by itself can flag a message as spam, the false positive rate can be greatly reduced. Outbound spam protection Outbound spam protection involves scanning email traffic as it exits a network, identifying spam messages and then taking an action such as blocking the message or shutting off the source of the traffic. While the primary impact of spam is on spam recipients, sending networks also experience financial costs, such as wasted bandwidth, and the risk of having their IP addresses blocked by receiving networks. Outbound spam protection not only stops spam, but also lets system administrators track down spam sources on their network and remediate them – for example, clearing malware from machines which have become infected with a virus or are participating in a botnet. PTR/reverse DNS checks The PTR DNS records in the reverse DNS can be used for a number of things, including: Most email mail transfer agents (mail servers) use a forward-confirmed reverse DNS (FCrDNS) verification and if there is a valid domain name, put it into the "Received:" trace header field. Some email mail transfer agents will perform FCrDNS verification on the domain name given in the SMTP HELO and EHLO commands. See #Strict enforcement of RFC standards §HELO/EHLO . To check the domain names in the rDNS to see if they are likely from dial-up users, dynamically assigned addresses, or home-based