Email filtering is the processing of

email Electronic mail (email or e-mail) is a method of exchanging messages ("mail") between people using electronic devices. Email was thus conceived as the electronic ( digital) version of, or counterpart to, mail, at a time when "mail" meant ...

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 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 typical ...

server, possibly applying

anti-spam techniques Various anti-spam techniques are used to prevent email spam (unsolicited bulk email). No technique is a complete solution to the spam problem, and each has trade-offs between incorrectly rejecting legitimate email (false positives) as opposed to ...

. Filtering can be applied to incoming emails as well as to outgoing ones. Depending on the calling environment, email filtering

software Software is a set of computer programs and associated documentation and data. This is in contrast to hardware, from which the system is built and which actually performs the work. At the lowest programming level, executable code consists ...

can reject an item at the initial SMTP connection stage or pass it through unchanged for delivery to the user's mailbox. It is also possible to redirect the message for delivery elsewhere, quarantine it for further checking, modify it or 'tag' it in any other way.

Motivation

Common uses for mail filters include organizing incoming email and removal of

spam Spam may refer to: * Spam (food), a canned pork meat product * Spamming, unsolicited or undesired electronic messages ** Email spam, unsolicited, undesired, or illegal email messages ** Messaging spam, spam targeting users of instant messaging ( ...

and

computer virus A computer virus is a type of computer program that, when executed, replicates itself by modifying other computer programs and inserting its own code. If this replication succeeds, the affected areas are then said to be "infected" with a compu ...

es. Mailbox providers filter outgoing email to promptly react to spam surges that may result from compromised accounts. A less common use is to inspect outgoing email at some companies to ensure that employees comply with appropriate policies and laws. Users might also employ a mail filter to prioritize messages, and to sort them into folders based on subject matter or other criteria.

Methods

Mailbox provider A mailbox provider, mail service provider or, somewhat improperly, email service provider is a provider of email hosting. It implements email servers to send, receive, accept, and store email for other organizations or end users, on their behalf ...

s can also install mail filters in their

mail transfer agent The mail or post is a system for physically transporting postcards, letters, and parcels. A postal service can be private or public, though many governments place restrictions on private systems. Since the mid-19th century, national postal syst ...

s as a service to all of their customers. Anti-virus, anti-spam, URL filtering, and authentication-based rejections are common filter types. Corporations often use filters to protect their employees and their

information technology Information technology (IT) is the use of computers to create, process, store, retrieve, and exchange all kinds of data . and information. IT forms part of information and communications technology (ICT). An information technology system (I ...

assets. A catch-all filter will "catch all" of the emails addressed to the domain that do not exist in the mail server - this can help avoid losing emails due to misspelling.

User Ancient Egyptian roles * User (ancient Egyptian official), an ancient Egyptian nomarch (governor) of the Eighth Dynasty * Useramen, an ancient Egyptian vizier also called "User" Other uses * User (computing), a person (or software) using an ...

s, may be able to install separate programs (see links below), or configure filtering as part of their

email program An email client, email reader or, more formally, message user agent (MUA) or mail user agent is a computer program used to access and manage a user's email. A web application which provides message management, composition, and reception functio ...

(''email client''). In email programs, users can make personal, "manual" filters that then automatically filter mail according to the chosen criteria.

Inbound and outbound filtering

Mail filters can operate on inbound and outbound email traffic. Inbound email filtering involves scanning messages from the Internet addressed to users protected by the filtering system or for

lawful interception Lawful interception (LI) refers to the facilities in telecommunications and telephone networks that allow law enforcement agencies with court orders or other legal authorization to selectively wiretap individual subscribers. Most countries require ...

. Outbound email filtering involves the reverse - scanning email messages from local users before any potentially harmful messages can be delivered to others on the Internet. One method of outbound email filtering that is commonly used by

Internet service provider An Internet service provider (ISP) is an organization that provides services for accessing, using, or participating in the Internet. ISPs can be organized in various forms, such as commercial, community-owned, non-profit, or otherwise private ...

s is

transparent SMTP proxy SMTP proxies are specialized mail transfer agents (MTAs) that, similar to other types of proxy servers, pass SMTP sessions through to other MTAs without using the store-and-forward approach of a typical MTA. When an SMTP proxy receives a connectio ...

ing, in which email traffic is intercepted and filtered via a transparent proxy within the network. Outbound filtering can also take place in an

email server Within the Internet email system, a message transfer agent (MTA), or mail transfer agent, or mail relay is software that transfers electronic mail messages from one computer to another using SMTP. The terms mail server, mail exchanger, and MX host ...

. Many corporations employ

data leak prevention Data loss prevention (DLP) software detects potential data breaches/data ex-filtration transmissions and prevents them by monitoring, detecting and blocking sensitive data while ''in use'' (endpoint actions), ''in motion'' (network traffic), and ' ...

technology in their outbound mail servers to prevent the leakage of sensitive information via email.

Customization

Mail filters have varying degrees of configurability. Sometimes they make decisions based on matching a

regular expression A regular expression (shortened as regex or regexp; sometimes referred to as rational expression) is a sequence of characters that specifies a search pattern in text. Usually such patterns are used by string-searching algorithms for "find" or ...

. Other times, code may match keywords in the message body, or perhaps the email address of the sender of the message. More complex

control flow In computer science, control flow (or flow of control) is the order in which individual statements, instructions or function calls of an imperative program are executed or evaluated. The emphasis on explicit control flow distinguishes an ''imper ...

and logic is possible with programming languages; this is typically implemented with a

data-driven programming In computer programming, data-driven programming is a programming paradigm in which the program statements describe the data to be matched and the processing required rather than defining a sequence of steps to be taken. Standard examples of dat ...

language, such as

procmail procmail is an email server software component — specifically, a message delivery agent (MDA). It was one of the earliest mail filter programs. It is typically used in Unix-like mail systems, using the mbox and Maildir storage formats. procm ...

, which specifies conditions to match and actions to take on matching, which may involve further matching. Some more advanced filters, particularly anti-spam filters, use statistical

document classification Document classification or document categorization is a problem in library science, information science and computer science. The task is to assign a document to one or more classes or categories. This may be done "manually" (or "intellectually") ...

techniques such as the

naive Bayes classifier In statistics, naive Bayes classifiers are a family of simple "probabilistic classifiers" based on applying Bayes' theorem with strong (naive) independence assumptions between the features (see Bayes classifier). They are among the simplest Baye ...

while others use

natural language processing Natural language processing (NLP) is an interdisciplinary subfield of linguistics, computer science, and artificial intelligence concerned with the interactions between computers and human language, in particular how to program computers to pro ...

to organize incoming emails. Image filtering can use complex image-analysis algorithms to detect skin-tones and specific body shapes normally associated with pornographic images.

Microsoft Outlook Microsoft Outlook is a personal information manager software system from Microsoft, available as a part of the Microsoft Office and Microsoft 365 software suites. Though primarily an email client, Outlook also includes such functions as Calen ...

includes user-generated email filters called "rules".

References

External links