a button on their email client which they can click to nominate a message as being spam); if the checksum is in the database, the message is likely to be spam. To avoid being detected in this way, spammers will sometimes insert unique invisible gibberish known as hashbusters into the middle of each of their messages, to make each message have a unique checksum. Country-based filtering Some email servers expect to never communicate with particular countries from which they receive a great deal of spam. Therefore, they use country-based filtering – a technique that blocks email from certain countries. This technique is based on country of origin determined by the sender's IP address rather than any trait of the sender. DNS-based blacklists There are large number of free and commercial DNS-based Blacklists, or DNSBLs which allow a mail server to quickly look up the IP of an incoming mail connection - and reject it if it is listed there. Administrators can choose from scores of DNSBLs, each of which reflects different policies: some list sites known to emit spam; others list open mail relays or proxies; others list ISPs known to support spam. URL filtering Most spam/phishing messages contain an URL that they entice victims into clicking on. Thus, a popular technique since the early 2000s consists of extracting URLs from messages and looking them up in databases such as Spamhaus' Domain Block List (DBL), SURBL, and URIBL. Strict enforcement of RFC standards Many spammers use poorly written software or are unable to comply with the standards because they do not have legitimate control of the computer they are using to send spam (zombie computer). By setting tighter limits on the deviation from RFC standards that the MTA will accept, a mail administrator can reduce spam significantly - but this also runs the risk of rejecting mail from older or poorly written or configured servers. Greeting delay – A sending server is required to wait until it has received the SMTP greeting banner before it sends any data. A deliberate pause can be introduced by receiving servers to allow them to detect and deny any spam-sending applications that do not wait to receive this banner. Temporary rejection – The greylisting technique is built on the fact that the SMTP protocol allows for temporary rejection of incoming messages. Greylisting temporarily rejects all messages from unknown senders or mail servers – using