Editing Email filtering

{{short description|Processing of email to organize it according to specified criteria}}
{{redirect|Spam filter|Wikipedia's spam filter|Wikipedia:Spam filter|selfref=yes}}
{{More citations needed |date=May 2024}}
{{Use dmy dates|date=September 2024}}
'''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 [[Simple Mail Transfer Protocol|SMTP]] server, possibly applying [[anti-spam techniques]]. Filtering can be applied to incoming emails as well as to outgoing ones.

Depending on the calling environment, email filtering [[software]] can reject an item at the initial SMTP connection stage<ref>With a [[List of SMTP server return codes#5xx Permanent errors|5xx SMTP status code]]</ref> 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 [[spamming|spam]] and [[computer virus]]es. Mailbox providers filter outgoing email to promptly react to spam surges that may result from compromised accounts.  A less common use is to [[Employee monitoring|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==
{{Main|Anti-spam techniques (users)|Anti-spam techniques|Email authentication}}
[[Mailbox provider]]s can also install mail filters in their [[mail transfer agent]]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]] 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 (computing)|User]]s, may be able to install separate programs (see links below), or configure filtering as part of their [[email client|email program]] (''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]]. 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.<ref>{{cite web|last=Zonk|title=How Pervasive is ISP Outbound Email Filtering?|url=http://ask.slashdot.org/story/08/01/31/2130251/How-Pervasive-is-ISP-Outbound-Email-Filtering|work=Slashdot.org|date=31 January 2008 |access-date=15 September 2011}}</ref> One method of outbound email filtering that is commonly used by [[Internet service provider]]s is [[transparent SMTP proxy]]ing, in which email traffic is intercepted and filtered via a transparent proxy within the network. Outbound filtering<ref>{{cite web|title=Inbound and Outbound Email Filtering|url=https://www.thexyz.com/email/spam-protection|work=thexyz.com|access-date=15 May 2023}}</ref> can also take place in an [[email server]]. Many corporations employ [[Data Leak Prevention|data leak prevention]] technology in their outbound [[Email server|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]]. 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]] and logic is possible with programming languages; this is typically implemented with a [[data-driven programming]] language, such as [[procmail]], 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]] techniques such as the [[naive Bayes classifier]] while others use [[natural language processing]] to organize incoming emails.<ref>{{Cite web|title=A Guide to Smart Filters {{!}} InMoat Knowledge Base|url=https://www.inmoat.com/knowledge-base-article/guide-to-smart-filters|access-date=19 October 2021|website=www.inmoat.com}}</ref> Image filtering can use complex image-analysis algorithms to detect skin-tones and specific body shapes normally associated with pornographic images.

[[Microsoft Outlook]] includes user-generated email filters called "rules".<ref>
{{cite book
| last1                 = Padwick
| first1                = Gordon
| last2                 = Feddema
| first2                = Helen Bell
| chapter               = 22: Creating and Using Rules
| title                 = Using Microsoft Outlook 2000
| url                   = https://archive.org/details/usingmicrosoftou00padw_0
| url-access            = registration
| series                = Using Series
| year = 1999
| location              = Indianapolis
| publisher             = Que Publishing
| publication-date      = 1999
| page                  = [https://archive.org/details/usingmicrosoftou00padw_0/page/618 618]
| isbn                  = 9780789719096
| access-date            = 10 January 2017
| quote                 = A rule is a set of conditions, actions, and exceptions that controls how Outlook processes and organizes messages. 
}}
</ref>

==See also==
* [[Bayesian spam filtering]]
* [[CRM114 (program)|CRM114]]
* [[Information filtering]]
* [[Markovian discrimination]]
* [[Anti-spam techniques#Outbound spam protection|Outbound Spam Protection]]
* [[Sieve (mail filtering language)]] is an RFC standard for describing mail filters
* [[White list#Email whitelists]]

==References==
{{Reflist}}

==External links==
{{spamming}}

[[Category:Communication software]]
[[Category:Email]]
[[Category:Anti-spam]]