An open mail relay is a Simple Mail Transfer Protocol (SMTP) server configured in such a way that it allows anyone on the Internet to send e-mail through it, not just mail destined to or originating from known users. This used to be the default configuration in many mail servers; indeed, it was the way the Internet was initially set up, but open mail relays have become unpopular because of their exploitation by spammers and worms. Many relays were closed, or were placed on blacklists by other servers.
A Domain Name System blocklist, Domain Name System-based blackhole list, Domain Name System blacklist (DNSBL) or real-time blackhole list (RBL) is a service for operation of mail servers to perform a check via a Domain Name System (DNS) query whether a sending host's IP address is blacklisted for email spam. Most mail server software can be configured to check such lists, typically rejecting or flagging messages from such sites.
A whitelist or allowlist is a list or register of entities that are being provided a particular privilege, service, mobility, access or recognition. Entities on the list will be accepted, approved and/or recognized. Whitelisting is the reverse of blacklisting, the practice of identifying entities that are denied, unrecognized, or ostracized.
Various anti-spam techniques are used to prevent email spam.
Email spam, also referred to as junk email, spam mail, or simply spam, is unsolicited messages sent in bulk by email (spamming). The name comes from a Monty Python sketch in which the name of the canned pork product Spam is ubiquitous, unavoidable, and repetitive. Email spam has steadily grown since the early 1990s, and by 2014 was estimated to account for around 90% of total email traffic.
Greylisting is a method of defending e-mail users against spam. A mail transfer agent (MTA) using greylisting will "temporarily reject" any email from a sender it does not recognize. If the mail is legitimate, the originating server will try again after a delay, and if sufficient time has elapsed, the email will be accepted.
Internet security is a branch of computer security. It encompasses the Internet, browser security, web site security, and network security as it applies to other applications or operating systems as a whole. Its objective is to establish rules and measures to use against attacks over the Internet. The Internet is an inherently insecure channel for information exchange, with high risk of intrusion or fraud, such as phishing, online viruses, trojans, ransomware and worms.
The Spamhaus Project is an international organisation based in the Principality of Andorra, founded in 1998 by Steve Linford to track email spammers and spam-related activity. The name spamhaus, a pseudo-German expression, was coined by Linford to refer to an internet service provider, or other firm, which spams or knowingly provides service to spammers.
PeerGuardian is a free and open source program developed by Phoenix Labs (software). It is capable of blocking incoming and outgoing connections based on IP blacklists. The aim of its use was to block peers on the same torrent download from any visibility of your own peer connection using IP lists. The system is also capable of blocking custom ranges, depending upon user preferences.
Email harvesting or scraping is the process of obtaining lists of email addresses using various methods. Typically these are then used for bulk email or spam.
SORBS was a list of e-mail servers suspected of sending or relaying spam. It had been augmented with complementary lists that include various other classes of hosts, allowing for customized email rejection by its users.
Barracuda Networks, Inc. provides security, networking and storage products based on network appliances and cloud services.
A challenge–response system is a type of that automatically sends a reply with a challenge to the (alleged) sender of an incoming e-mail. It was originally designed in 1997 by Stan Weatherby, and was called Email Verification. In this reply, the purported sender is asked to perform some action to assure delivery of the original message, which would otherwise not be delivered. The action to perform typically takes relatively little effort to do once, but great effort to perform in large numbers. This effectively filters out spammers. Challenge–response systems only need to send challenges to unknown senders. Senders that have previously performed the challenging action, or who have previously been sent e-mail(s) to, would be automatically receive a challenge.
SURBL is a collection of URI DNSBL lists of Uniform Resource Identifier (URI) hosts, typically web site domains, that appear in unsolicited messages. SURBL can be used to search incoming e-mail message bodies for spam payload links to help evaluate whether the messages are unsolicited. For example, if http://www.example.com is listed, then e-mail messages with a message body containing this URI may be classified as unsolicited. URI DNSBLs differ from prior DNSBLs, which commonly list mail sending IP addresses. SURBL is a specific instance of the general URI DNSBL list type.
The Mail Abuse Prevention System (MAPS) is an organization that provides anti-spam support by maintaining a DNSBL. They provide five black lists, categorising why an address or an IP block is listed:
Fail2Ban is an intrusion prevention software framework. Written in the Python programming language, it is designed to prevent brute-force attacks. It is able to run on POSIX systems that have an interface to a packet-control system or firewall installed locally, such as iptables or TCP Wrapper.
Srizbi BotNet is considered one of the world's largest botnets, and responsible for sending out more than half of all the spam being sent by all the major botnets combined. The botnets consist of computers infected by the Srizbi trojan, which sent spam on command. Srizbi suffered a massive setback in November 2008 when hosting provider Janka Cartel was taken down; global spam volumes reduced up to 93% as a result of this action.
Forum spam consists of posts on Internet forums that contains related or unrelated advertisements, links to malicious websites, trolling and abusive or otherwise unwanted information. Forum spam is usually posted onto message boards by automated spambots or manually with unscrupulous intentions with intent to get the spam in front of readers who would not otherwise have anything to do with it intentionally.
SmartScreen is a cloud-based anti-phishing and anti-malware component included in several Microsoft products:
Dorkbot is a family of malware worms that spreads through instant messaging, USB drives, websites or social media channels like Facebook. Code Shikara is a computer worm, related to the Dorkbot family, that attacks through social engineering. Particularly prevalent in 2015, Dorkbot-infected systems were variously used to send spam, participate in DDoS attacks, or harvest users' credentials.
