Editing Email (section)

==Operation==
The following is a typical sequence of events that takes place when sender [[Placeholder names in cryptography|Alice]] transmits a message using a [[E-mail client|mail user agent]] (MUA) addressed to the [[email address]] of the recipient.<ref>{{cite video|title=How E-mail Works|publisher=howstuffworks.com|year=2008|url=https://computer.howstuffworks.com/e-mail-messaging/email.htm|url-status=live|archive-url=https://web.archive.org/web/20170611181852/https://computer.howstuffworks.com/e-mail-messaging/email.htm|archive-date=2017-06-11}}</ref>

[[File:email.svg|thumb|Email operation]]
# The MUA formats the message in email format and uses the submission protocol, a profile of the [[Simple Mail Transfer Protocol]] (SMTP), to send the message content to the local [[mail submission agent]] (MSA), in this case ''smtp.a.org''.
# The MSA determines the destination address provided in the SMTP protocol (not from the message header)—in this case, ''bob@b.org''—which is a [[Fully qualified domain address|fully qualified domain address (FQDA)]]. The part before the @ sign is the ''local part'' of the address, often the [[username]] of the recipient, and the part after the @ sign is a [[domain name]]. The MSA resolves a domain name to determine the [[fully qualified domain name]] of the [[Message transfer agent|mail server]] in the [[Domain Name System]] (DNS).
# The [[DNS server]] for the domain ''b.org'' (''ns.b.org'') responds with any [[MX record]]s listing the mail exchange servers for that domain, in this case ''mx.b.org'', a [[message transfer agent]] (MTA) server run by the recipient's ISP.<ref>[https://dnsdb.cit.cornell.edu/explain_mx.html "MX Record Explanation"] {{webarchive|url=https://web.archive.org/web/20150117204846/https://dnsdb.cit.cornell.edu/explain_mx.html |date=2015-01-17 }}, it.cornell.edu</ref>
# smtp.a.org sends the message to mx.b.org using SMTP. This server may need to forward the message to other MTAs before the message reaches the final [[message delivery agent]] (MDA).
# The MDA delivers it to the [[Email Mailbox|mailbox]] of user ''bob''.
# Bob's MUA picks up the message using either the [[Post Office Protocol]] (POP3) or the [[Internet Message Access Protocol]] (IMAP).

In addition to this example, alternatives and complications exist in the email system:
* Alice or Bob may use a client connected to a corporate email system, such as [[IBM]] [[Lotus Notes]] or [[Microsoft]] [[Microsoft Exchange Server|Exchange]]. These systems often have their own internal email format and their clients typically communicate with the email server using a vendor-specific, proprietary protocol. The server sends or receives email via the Internet through the product's Internet mail gateway which also does any necessary reformatting. If Alice and Bob work for the same company, the entire transaction may happen completely within a single corporate email system.
* Alice may not have an MUA on her computer but instead may connect to a [[webmail]] service.
* Alice's computer may run its own MTA, so avoiding the transfer at step 1. 
* Bob may pick up his email in many ways, for example logging into mx.b.org and reading it directly, or by using a webmail service.
* Domains usually have several mail exchange servers so that they can continue to accept mail even if the primary is not available.

Many MTAs used to accept messages for any recipient on the Internet and do their best to deliver them. Such MTAs are called ''[[open mail relay]]s''. This was very important in the early days of the Internet when network connections were unreliable.<ref>{{cite web|url=https://searchnetworking.techtarget.com/sDefinition/0,,sid7_gci782509,00.html |title=What is open relay? |access-date=2008-04-07 |date=2004-07-19 |work=WhatIs.com |publisher=[[Indiana University]] |archive-url=https://web.archive.org/web/20070824005337/https://searchnetworking.techtarget.com/sDefinition/0,,sid7_gci782509,00.html |archive-date=2007-08-24 }}</ref><ref>{{cite book|author=Ch Seetha Ram|title=Information Technology for Management|url=https://books.google.com/books?id=0bHAGrUxqRsC&pg=PA164|year=2010|publisher=Deep & Deep Publications|isbn=978-81-8450-267-1|page=164}}</ref>  However, this mechanism proved to be exploitable by originators of [[email spam|unsolicited bulk email]] and as a consequence open mail relays have become rare,<ref>{{cite web|url=https://www.imc.org/ube-relay.html |title=Allowing Relaying in SMTP: A Series of Surveys |access-date=2008-04-13 |last=Hoffman |first=Paul |date=2002-08-20 |work=IMC Reports |publisher=[[Internet Mail Consortium]] |archive-url=https://web.archive.org/web/20070118121843/https://www.imc.org/ube-relay.html |archive-date=2007-01-18 }}</ref> and many MTAs do not accept messages from open mail relays.

Email pre-dates [[instant messaging]], and transmission favors reliability over speed, in order to be able to cope with unreliable network links and busy servers (more common in the early days of the Internet). Reasons for slower delivery include:<ref name="luxsci">[https://luxsci.com/blog/why-email-is-not-instantaneous-and-not-supposed-to-be.html Why Email is Not Instantaneous — and Not Supposed to Be]</ref>
* Messages going to a large number of recipients require more processing
* Large messages (e.g. with large attachments) require more time to transmit over the network
* Messages need to pass through multiple servers (sometimes multiple servers inside the same organization)
* One or more mail servers are overloaded (possibly due to [[email spam|spam]] or [[denial-of-service attack]]) and queuing incoming mail or temporarily refusing incoming connections
* [[SMTP]] requires multiple back-and-forths, which can amplify the impact of a slow network or dropped packets
* Sender or recipient temporarily disconnected from the network (e.g. a laptop out of wifi range)
* Slow DNS response
* Server down for maintenance or malfunction

Mail can be queued and retried for up to five days before senders are notified of a permanent delivery failure.<ref name="luxsci" /> Messages are timestamped as they pass through each server, allowing for diagnosis of slow delivery, though analysis is complicated by [[time zone]]s and computer clocks that are inaccurately set.<ref name="luxsci" />

Email messages classified as [[email spam|spam]] by a [[spam filter]] may be sorted into a separate folder which the recipient must check manually, or may be dropped entirely.