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Electronic mail is a method of transmitting and receiving messages using electronic devices. It was conceived in the late–20th century as the digital version of, or counterpart to, mail. Email is a ubiquitous and very widely used communication medium; in current use, an email address is often treated as a basic and necessary part of many processes in business, commerce, government, education, entertainment, and other spheres of daily life in most countries.
The Simple Mail Transfer Protocol (SMTP) is an Internet standard communication protocol for electronic mail transmission. Mail servers and other message transfer agents use SMTP to send and receive mail messages. User-level email clients typically use SMTP only for sending messages to a mail server for relaying, and typically submit outgoing email to the mail server on port 587 or 465 per RFC 8314. For retrieving messages, IMAP is standard, but proprietary servers also often implement proprietary protocols, e.g., Exchange ActiveSync.
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.
The Spam Prevention Early Warning System (SPEWS) was an anonymous service that maintained a list of IP address ranges belonging to internet service providers (ISPs) that host spammers and show little action to prevent their abuse of other networks' resources. It could be used by Internet sites as an additional source of information about the senders of unsolicited bulk email, better known as spam.
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.
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.
Sender Policy Framework (SPF) is an email authentication method which ensures the sending mail server is authorized to originate mail from the email sender's domain. This authentication only applies to the email sender listed in the "envelope from" field during the initial SMTP connection. If the email is bounced, a message is sent to this address, and for downstream transmission it typically appears in the "Return-Path" header. To authenticate the email address which is actually visible to recipients on the "To:" line, other technologies such as DMARC must be used. Forgery of this address is known as email spoofing, and is often used in phishing and email spam.
A bounce message or just "bounce" is an automated message from an email system, informing the sender of a previous message that the message has not been delivered. The original message is said to have "bounced".
Email filtering is the processing of email to organize it according to specified criteria. The term can apply to the intervention of human intelligence, but most often refers to the automatic processing of messages at an SMTP server, possibly applying anti-spam techniques. Filtering can be applied to incoming emails as well as to outgoing ones.
On-Demand Mail Relay (ODMR) is an SMTP extension standardized in RFC 2645 that allows e-mail to be relayed to the recipients after they have been authenticated. It uses the extended SMTP command ATRN, similar to the ETRN command but available for dynamically assigned IP addresses.
Email authentication, or validation, is a collection of techniques aimed at providing verifiable information about the origin of email messages by validating the domain ownership of any message transfer agents (MTA) who participated in transferring and possibly modifying a message.
A message submission agent (MSA), or mail submission agent, is a computer program or software agent that receives electronic mail messages from a mail user agent (MUA) and cooperates with a mail transfer agent (MTA) for delivery of the mail. It uses ESMTP, a variant of the Simple Mail Transfer Protocol (SMTP), as specified in RFC 6409.
Forward-confirmed reverse DNS (FCrDNS), also known as full-circle reverse DNS, double-reverse DNS, or iprev, is a networking parameter configuration in which a given IP address has both forward (name-to-address) and reverse (address-to-name) Domain Name System (DNS) entries that match each other. This is the standard configuration expected by the Internet standards supporting many DNS-reliant protocols. David Barr published an opinion in RFC 1912 (Informational) recommending it as best practice for DNS administrators, but there are no formal requirements for it codified within the DNS standard itself.
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:
Email forwarding generically refers to the operation of re-sending a previously delivered email to an email address to one or more different email addresses.
An email alias is simply a forwarding email address. The term alias expansion is sometimes used to indicate a specific mode of email forwarding, thereby implying a more generic meaning of the term email alias as an address that is forwarded in a simplistic fashion.
Backscatter is incorrectly automated bounce messages sent by mail servers, typically as a side effect of incoming spam.
SMTP Authentication, often abbreviated SMTP AUTH, is an extension of the Simple Mail Transfer Protocol (SMTP) whereby a client may log in using any authentication mechanism supported by the server. It is mainly used by submission servers, where authentication is mandatory.
People tend to be much less bothered by spam slipping through filters into their mail box, than having desired e-mail ("ham") blocked. Trying to balance false negatives vs false positives is critical for a successful anti-spam system. As servers are not able to block all spam there are some tools for individual users to help control over this balance.
References
- 1 2 3 Moen, Rick (June 2005). "Mail Relaying/Authentication". Linux Gazette (115).
The term 'smarthost' harks back to the days before SMTP was universal and DNS reasonably reliable: The idea was that you assumed that your own host probably wasn't as well-connected and capable as, say, the UCBVAX machine at Berkeley. So, you would configure your local outbound mail processes to lob all outgoing mail over to UCBVAX via batched, dialed-up UUCP, or SMTP, or whatever, trusting to UCBVAX to perform necessary DNS lookups and redeliver the mail. UCBVAX was, in that sense, dubbed a "smarthost" for your site, in the sense that it's smarter about DNS and mail connections than yours is.
- ↑ John, Chris. "When Using a Smart Host is the Smart Choice". SocketLabs. Retrieved 19 December 2018.
- ↑ "Getting past email blocking". The National Bureau of Economic Research. Retrieved 19 December 2018.