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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.
The initiating host, the SMTP client, can be either an end-user's email client, functionally identified as a mail user agent (MUA), or a relay server's mail transfer agent (MTA), that is an SMTP server acting as an SMTP client, in the relevant session, in order to relay mail. Fully capable SMTP servers maintain queues of messages for retrying ...
This is a list of Simple Mail Transfer Protocol (SMTP) response status codes. Status codes are issued by a server in response to a client's request made to the server. Unless otherwise stated, all status codes described here is part of the current SMTP standard, RFC 5321. The message phrases shown are typical, but any human-readable alternative ...
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.
Within the Internet email system, a message transfer agent (MTA), [1] mail transfer agent, [2] or mail relay is software that transfers electronic mail messages from one computer to another using the Simple Mail Transfer Protocol. [3] In some contexts, the alternative names mail server, mail exchanger, or MX host are used to describe an MTA.
The comparison of mail servers covers mail transfer agents (MTAs), mail delivery agents, and other computer software that provide e-mail services. Unix -based mail servers are built using a number of components because a Unix-style environment is, by default, a toolbox [ 1 ] operating system.
In the early 1980s, when Simple Mail Transfer Protocol (SMTP) was designed, it provided for no real verification of sending user or system. This was not a problem while email systems were run by trusted corporations and universities, but since the commercialization of the Internet in the early 1990s, spam, phishing, and other crimes have been found to increasingly involve email.
A smart host or smarthost is an email server via which third parties can send emails and have them forwarded on to the email recipients' email servers. [1]Smarthosts were originally open mail relays, [1] but most providers now require authentication from the sender, to verify that the sender is authorised [1] – for example, an ISP might run a smarthost for their paying customers only.