The Internet is a global system of interconnected computer networks that use the standard Internet protocol suite to serve billions of users worldwide...
message handling services (MHS), a message transfer agent
or mail transfer agent
) or mail relay
is software that transfers electronic mail messages from one computer to another using a client–server application architecture. An MTA implements both the client (sending) and server (receiving) portions of the Simple Mail Transfer Protocol
Simple Mail Transfer Protocol is an Internet standard for electronic mail transmission across Internet Protocol networks. SMTP was first defined by RFC 821 , and last updated by RFC 5321 which includes the extended SMTP additions, and is the protocol in widespread use today...
The terms mail server
, mail exchanger
, and MX host
may also refer to a computer
A computer is a programmable machine designed to sequentially and automatically carry out a sequence of arithmetic or logical operations. The particular sequence of operations can be changed readily, allowing the computer to solve more than one kind of problem...
performing the MTA function. The Domain Name System
The Domain Name System is a hierarchical distributed naming system for computers, services, or any resource connected to the Internet or a private network. It associates various information with domain names assigned to each of the participating entities...
(DNS) associates a mail server to a domain with mail exchanger
A mail exchanger record is a type of resource record in the Domain Name System that specifies a mail server responsible for accepting email messages on behalf of a recipient's domain, and a preference value used to prioritize mail delivery if multiple mail servers are available...
(MX) resource records containing the domain name
A domain name is an identification string that defines a realm of administrative autonomy, authority, or control in the Internet. Domain names are formed by the rules and procedures of the Domain Name System ....
of a host providing MTA services.
A mail server is a computer that serves as an electronic post office for email. Mail exchanged across networks is passed between mail servers that run specially designed software. This software is built around agreed-upon, standardized protocols for handling mail messages and the graphics they might contain.
A message transfer agent receives mail from either another MTA, a mail submission agent
A mail submission agent is a computer program or software agent that receives electronic mail messages from a mail user agent and cooperates with a mail transfer agent for delivery of the mail...
(MSA), or a mail user agent (MUA). The transmission details are specified by the Simple Mail Transfer Protocol (SMTP). When a recipient mailbox of a message is not hosted locally, the message is relayed, that is, forwarded to another MTA. Every time an MTA receives an email message, it adds a Received
trace header field to the top of the header of the message, thereby building a sequential record of MTAs handling the message. The process of choosing a target MTA for the next hop is also described in SMTP, but can usually be overridden by configuring the MTA software with specific routes.
A MTA works in the background, while the user usually interacts directly with a mail user agent. One may distinguish initial submission as first passing through an MSA – port 587 is used for communication between an MUA and an MSA while port 25 is used for communication between MTAs, or from an MSA to an MTA; this distinction is first made in RFC 2476.
For recipients hosted locally, the final delivery of email to a recipient mailbox is the task of a message delivery agent (MDA). For this purpose the MTA transfers the message to the message handling service component of the message delivery agent. Upon final delivery, the Return-Path
field is added to the envelope to record the return path.
Transfer versus access
The function of an MTA is usually complemented with some means for email clients to access stored messages. This function typically employs a different protocol. The most widely implemented open protocols for the MUA are the Post Office Protocol
In computing, the Post Office Protocol is an application-layer Internet standard protocol used by local e-mail clients to retrieve e-mail from a remote server over a TCP/IP connection. POP and IMAP are the two most prevalent Internet standard protocols for e-mail retrieval. Virtually all modern...
(POP3) and the Internet Message Access Protocol
Internet message access protocol is one of the two most prevalent Internet standard protocols for e-mail retrieval, the other being the Post Office Protocol...
(IMAP), but many proprietary systems exist (Exchange
Microsoft Exchange Server is the server side of a client–server, collaborative application product developed by Microsoft. It is part of the Microsoft Servers line of server products and is used by enterprises using Microsoft infrastructure products...
Lotus Notes is the client of a collaborative platform originally created by Lotus Development Corp. in 1989. In 1995 Lotus was acquired by IBM and became known as the Lotus Development division of IBM and is now part of the IBM Software Group...
Domino/Notes) for retrieving messages. Many systems also offer a web interface for reading and sending email that is independent of any particular MUA.
At its most basic, an MUA using POP3 downloads messages from the server mailbox onto the local computer for display in the MUA. Messages are generally removed from the server at the same time but most systems also allow a copy to be left behind as a backup. In contrast, an MUA using IMAP displays messages directly from the server, although a download option for archive purposes is usually also available. One advantage this gives IMAP is that the same messages are visible from any computer accessing the email account, since messages aren't routinely downloaded and deleted from the server. If set up properly, sent mail can be saved to the server also, in contrast with POP mail, where sent messages exist only in the local MUA and are not visible by other MUAs accessing the same account.
The IMAP protocol has features that allow uploading of mail messages and there are implementations that can be configured to also send
messages like an MTA, which combine sending a copy and storing a copy in the Sent
folder in one upload operation.
The reason for using SMTP as a standalone transfer protocol is twofold:
- To cope with discontinuous connections. Historically, inter-network connections were not continuously available as they are today and many readers didn't need an access protocol, as they could access their mailbox
An email box is the equivalent of a letter box for electronic mail. It is the destination where electronic mail messages are delivered.-Definitions:...
directly (as a file) through a terminal connection. SMTP, if configured to use backup MXes, can transparently cope with temporary local network outages. A message can be transmitted along a variable path by choosing the next hop from a preconfigured list of MXes with no intervention from the originating user.
- Submission policies. Modern systems are designed for users to submit messages to their local servers for policy, not technical, reasons. It was not always that way. For example, the original Eudora
Eudora is an e-mail client used on the Apple Macintosh and Microsoft Windows operating systems. It also supports several palmtop computing platforms, including Newton and the Palm OS....
email client featured direct delivery of mail to the recipients' servers, out of necessity. Today, funneling email through MSA systems run by providers that in principle have some means of holding their users accountable for the generation of the email is a defense against spam and other forms of email abuse.
Issues affecting small email-servers
In recent years, mainly due to concerns over spam
Spam may refer to:* Spam , a canned pork meat product* Spam , unsolicited or undesired electronic messages* E-mail spam, unsolicited or undesired email messages* "Spam" , a comedy sketch...
and a general trend towards centralisation, problems have arisen for small organisations and home users wishing to run their own email server. many ISPs pre-emptively block outgoing connections to TCP port 25 on domestic connections, and larger email providers have increasingly stringent requirements for other servers that wish to transfer emails to them. For example: reverse PTR records of the sending mail server are often checked before accepting mail. The PTR record must be set up by the ISP, which may refuse this request to a small-business or domestic user.
Other problems encountered by small mail-servers include zealous use of blacklisting and a presumption of guilt by blacklisting services and large email providers, which classify "new" servers as spammers by default. Such measures have inevitably reduced the overall number of small email-servers, and some end-users have opted to outsource to paid services or to GMail
Gmail is a free, advertising-supported email service provided by Google. Users may access Gmail as secure webmail, as well via POP3 or IMAP protocols. Gmail was launched as an invitation-only beta release on April 1, 2004 and it became available to the general public on February 7, 2007, though...
instead, exacerbating the problem for those not wishing to outsource.
List of MTA software for Unix-like operating systems
- Apache James
Apache James, aka Java Apache Mail Enterprise Server or some variation thereof, is an open source SMTP and POP3 mail transfer agent and NNTP news server written entirely in Java. James is maintained by contributors to the Apache Software Foundation, with initial contributions by Serge Knystautas....
- Courier Mail Server
The Courier mail server is a mail transfer agent server that provides ESMTP, IMAP, POP3, SMAP, webmail, and mailing list services with individual components. It is best known for its IMAP server component....
- Dragonfly Mail Agent
Exim is a mail transfer agent used on Unix-like operating systems. Exim is free software distributed under the terms of the GNU General Public License, and it aims to be a general and flexible mailer with extensive facilities for checking incoming e-mail....
The MagicMail email server is a turn-key carrier grade email platform designed specifically for the tier-2 Internet service provider and telco market. It is built to run on a custom LinuxMagic-built operating system based on Linux. MagicMail utilizes open source technologies blended with...
MMDF, the Multichannel Memorandum Distribution Facility, is a message transfer agent , a computer program designed to transmit email.-History:...
In computing, Postfix is a free and open-source mail transfer agent that routes and delivers electronic mail. It is intended as a fast, easier-to-administer, and secure alternative to the widely-used Sendmail MTA....
qmail is a mail transfer agent that runs on Unix. It was written, starting December 1995, by Daniel J. Bernstein as a more secure replacement for the popular Sendmail program...
qpsmtpd is an SMTP daemon written in Perl. It was originally designed to be a drop-in replacement for qmail-smtpd, the SMTP component of qmail, and it is now also compatible with Postfix, Exim, sendmail and virtually any software that "speaks SMTP"...
Sendmail is a general purpose internetwork email routing facility that supports many kinds of mail-transfer and -delivery methods, including the Simple Mail Transfer Protocol used for email transport over the Internet....
Smail-3 is a mail transfer agent used on Unix-like operating systems. It is freely available but is not a package under the GNU GPL. It aims to be a general and flexible mailer with extensive facilities for checking incoming e-mail...
- Synovel (email server)
Synovel Collabsuite is an open source Linux mail server with calendaring and chat features . The server also includes anti-spam, antivirus, email archival.User can access the features through Desktop client, Web or Mobile phone.- Features :...
Zimbra Collaboration Suite is a groupware product created by Zimbra, Inc., located in Palo Alto, California, USA. The company was purchased by Yahoo! in September 2007, and subsequently purchased by VMware on Tuesday, January 12, 2010. The software consists of both client and server components...
ZMailer is a mail transfer agent for Linux, BSD and other Unix-like systems.It is intended for gateways or mail servers or other large site environments that have extreme demands on the abilities of the mailer....
- Comparison of mail servers
This is a comparison of mail servers: mail transfer agents, mail delivery agents, and other computer software which provide e-mail services.- Feature comparison :- Authentication :- Antispam Features :...
- E-mail agent (infrastructure) (MxA)
- Email forwarding
Email forwarding generically refers to the operation of re-sending an email message delivered to one email address on to a possibly different email address...
- List of mail servers
- SMTP proxy