Terminology and Concepts
Definitions and Etymology
Email, short for electronic mail, refers to the exchange of digital messages between computer users over a communications network, such as the Internet, using standardized protocols to ensure reliable delivery and retrieval.[4] This system enables asynchronous communication, where messages are stored on servers until the recipient accesses them, distinguishing it from real-time methods.[8] The term "electronic mail" originated in the early 1970s amid the rise of networked computing, with Ray Tomlinson credited for implementing the first networked email system in 1971 while working on ARPANET.[9] The abbreviation "email" (or "e-mail") first appeared in print in 1979 and became common in the 1980s, reflecting the technology's evolution from time-sharing systems to widespread internet use.[10] Developers chose "electronic mail" to parallel traditional postal services, emphasizing structured delivery and addressing to make the concept accessible to non-technical users. Email differs from postal mail, which relies on physical transport and can take days, by providing near-instantaneous digital transmission without tangible media.[8] In contrast to instant messaging, which supports synchronous, conversation-like exchanges often requiring both parties to be online simultaneously, email allows deferred reading and attachment of files or multimedia.[11] Short Message Service (SMS), meanwhile, is constrained to brief texts via cellular networks, lacking email's capacity for complex formatting or long-form content.[12] Many email-specific terms draw from postal analogies to aid user familiarity. "Inbox" and "outbox" mimic physical trays for incoming and outgoing correspondence in office mailrooms, a convention established in early email software to simulate interoffice memo handling.[13] The term "spam," denoting unsolicited bulk messages, stems from a 1970 Monty Python comedy sketch where the word "Spam" is repeated incessantly, a metaphor first applied to disruptive online posts in 1980s multi-user dungeons (MUDs) and later to email in the early 1990s.[14]Core Components
An email system relies on several interconnected core components to facilitate the composition, submission, transfer, storage, and retrieval of messages. At its foundation, the sender's device hosts a Message User Agent (MUA), which serves as the interface for composing and submitting emails. This client software interacts with a Mail Submission Agent (MSA) to initiate the process, ensuring messages are properly formatted and authenticated before entering the network.[8] On the recipient side, a corresponding Recipient MUA (rMUA) enables the viewing and management of incoming messages, typically after retrieval from a remote server.[8] Central to the system's operation are the message servers, which handle the intermediary roles of transfer and delivery. Message Transfer Agents (MTAs) act as relays, routing emails across networks by examining addresses and forwarding them hop-by-hop without modifying the content, except for adding trace information. Mail Delivery Agents (MDAs), in turn, perform the final deposition of messages into designated storage. These server-based components operate within Administrative Management Domains (ADMDs), distinguishing local ecosystems—such as intra-organizational handling within a single domain—from remote ones that span multiple domains and require boundary relays for secure handoff.[8] Email addresses play a pivotal role in this architecture, structured in the format<local-part>@<domain>, where the local-part identifies a specific mailbox within the domain, and the domain specifies the responsible ADMD for routing and delivery. Domains are resolved globally via the Domain Name System (DNS), enabling accurate navigation through the decentralized network. Mailboxes function as digital repositories within a Message Store (MS) on the recipient's server, holding incoming messages until accessed by the rMUA; they also support storage for outgoing drafts or sent items on the sender's side. This separation underscores the conceptual divide between local components, like an individual's MUA on their device, and remote ones, such as MTAs and MDAs hosted on infrastructure servers.[8]