A message submission agent (MSA), or mail submission agent, is a
computer program
A computer program is a sequence or set of instructions in a programming language for a computer to execute. Computer programs are one component of software, which also includes documentation and other intangible components.
A computer program ...
or
software agent
In computer science, a software agent or software AI is a computer program that acts for a user or other program in a relationship of agency, which derives from the Latin ''agere'' (to do): an agreement to act on one's behalf. Such "action on beha ...
that receives
electronic mail
Electronic mail (email or e-mail) is a method of exchanging messages ("mail") between people using electronic devices. Email was thus conceived as the electronics, electronic (digital media, digital) version of, or counterpart to, mail, ...
messages from a
mail user agent
The mail or post is a system for physically transporting postcards, letters, and parcels. A postal service can be private or public, though many governments place restrictions on private systems. Since the mid-19th century, national postal syst ...
(MUA) and cooperates with a
mail transfer agent
The mail or post is a system for physically transporting postcards, letters, and parcels. A postal service can be private or public, though many governments place restrictions on private systems. Since the mid-19th century, national postal syst ...
(MTA) for delivery of the mail. It uses ESMTP, a variant of the
Simple Mail Transfer Protocol
The Simple Mail Transfer Protocol (SMTP) is an Internet standard communication protocol for electronic mail transmission. Mail servers and other message transfer agents use SMTP to send and receive mail messages. User-level email clients typica ...
(SMTP), as specified in
RFC
RFC may refer to:
Computing
* Request for Comments, a memorandum on Internet standards
* Request for change, change management
* Remote Function Call, in SAP computer systems
* Rhye's and Fall of Civilization, a modification for Sid Meier's Civ ...
6409.
Many MTAs perform the function of an MSA as well, but there are also programs that are specially designed as MSAs without full MTA functionality. Historically, in
Internet mail
Electronic mail (email or e-mail) is a method of exchanging messages ("mail") between people using electronic devices. Email was thus conceived as the electronic ( digital) version of, or counterpart to, mail, at a time when "mail" meant ...
, both MTA and MSA functions use
port number
In computer networking, a port is a number assigned to uniquely identify a connection endpoint and to direct data to a specific service. At the software level, within an operating system, a port is a logical construct that identifies a specific ...
25, but the official port for MSAs is 587.
[
] The MTA accepts a user's incoming mail, while the MSA accepts a user's outgoing mail.
Benefits
Separation of the
MTA and MSA functions produces several benefits.
One benefit is that an MSA, since it is interacting directly with the author's MUA, can correct minor errors in a message's format (such as a missing ''Date'', ''
Message-ID
Message-ID is a unique identifier for a digital message, most commonly a globally unique identifier used in email and Usenet newsgroups.
Message-IDs are required to have a specific format which is a subset of an email address
An email address id ...
'', ''To'' fields, or an address with a missing domain name) and/or immediately report an error to the author so that it can be corrected before it is sent to any of the recipients. An MTA accepting a message from another site cannot reliably make those kinds of corrections, and any error reports generated by such an MTA will reach the author (if at all) only after the message has already been sent.
One more benefit is that with a dedicated port number, 587, it is always possible for users to connect to ''their domain'' to submit new mail. To combat spam (including spam being sent unwittingly by a victim of a
botnet
A botnet is a group of Internet-connected devices, each of which runs one or more bots. Botnets can be used to perform Distributed Denial-of-Service (DDoS) attacks, steal data, send spam, and allow the attacker to access the device and its conn ...
) many
ISPs
An Internet service provider (ISP) is an organization that provides services for accessing, using, or participating in the Internet. ISPs can be organized in various forms, such as commercial, community-owned, non-profit, or otherwise private ...
and institutional
networks
Network, networking and networked may refer to:
Science and technology
* Network theory, the study of graphs as a representation of relations between discrete objects
* Network science, an academic field that studies complex networks
Mathematics
...
restrict the ability to connect to remote MTAs on port 25. The accessibility of an MSA on port 587 enables nomadic users (for example, those working on a laptop) to continue to send mail via their preferred submission servers even from within others' networks. Using a specific submission server is a requirement when
sender policies or
signing practices are enforced.
Another benefit is that separating the MTA and MSA functions makes it easier for an MTA to deny relaying, that is to refuse any mail that is not addressed to a recipient at a domain that is served locally. This is a strategy used by ISPs to prevent the sending of spam from virus-infected client computers. By contrast, an MSA must generally accept mail for any recipient on the Internet, though it only accepts such mail from authors who are authorized to use that MSA and who have established their identity to the MSA via authentication. In times when both mail submission and acceptance of incoming mail were usually accomplished using the same protocol and the same server, the ability to send mail to arbitrary destinations without authentication allowed spammers to use MTAs as a means of distributing
spam
Spam may refer to:
* Spam (food), a canned pork meat product
* Spamming, unsolicited or undesired electronic messages
** Email spam, unsolicited, undesired, or illegal email messages
** Messaging spam, spam targeting users of instant messaging ( ...
(since a single message transaction can request that an MTA relay a message to a large number of recipients), and also made it more difficult to trace a message to its origin.
Yet another benefit is that MSAs and MTAs can have different policies for filtering of spam. Most MSAs require authentication in the form of a username and password provided by the author. Any messages received by such an MSA are therefore traceable to an author who has a direct relationship with the MSA, and who can be held accountable for his actions. This allows the MSA to have either no spam filtering, or more permissive spam filtering than an MTA that exists for the purpose of accepting incoming email from other domains. It is difficult to establish trust in mail sent between arbitrary domains, because there is generally no direct relationship between those domains via which trust, or even identity, can be established. In the absence of such trust, an MTA must generally rely on heuristics and third-party reputation services to distinguish spam from legitimate traffic, and both of these mechanisms have a history of being error-prone. The separation of MSA and MTA therefore avoids the use of unreliable spam recognition mechanisms during mail submission, and increases the probability for legitimate mail to be delivered successfully.
Protocol
Configuration
While recent
email client
An email client, email reader or, more formally, message user agent (MUA) or mail user agent is a computer program used to access and manage a user's email.
A web application which provides message management, composition, and reception functio ...
s use port 587 by default, older ones still propose port 25. Users have to change the port number manually in the latter case. It is also possible that the MUA may automatically discover which server provides the MSA for a given domain, looking up the
SRV record
A Service record (SRV record) is a specification of data in the Domain Name System defining the location, i.e., the hostname and port number, of servers for specified services. It is defined iRFC 2782 and its type code is 33. Some Internet protoco ...
s for that domain. Domain example.com can publish its record like so:
_submission._tcp.example.com. SRV 0 1 587 mail.example.com.
Mandatory authentication
RFC 6409 requires that clients are authorized and authenticated to use the mail submission service, e.g., as described in SMTP-AUTH (ESMTPA), or by other means such as
RADIUS
In classical geometry, a radius ( : radii) of a circle or sphere is any of the line segments from its center to its perimeter, and in more modern usage, it is also their length. The name comes from the latin ''radius'', meaning ray but also the ...
,
public key certificate
In cryptography, a public key certificate, also known as a digital certificate or identity certificate, is an electronic document used to prove the validity of a public key. The certificate includes information about the key, information about the ...
s, or (the mostly obsolete)
POP before SMTP
POP before SMTP or SMTP after POP is a method of authentication used by mail server software which helps allow users the option to send e-mail from any location, as long as they can demonstrably also fetch their mail from the same place.
The POP b ...
.
Policy enforcement
The MSA must check that the submitted mail is syntactically valid and conforms to the relevant site policies. RFC 6409 contains some optional features:
* ''Enforce submission rights'' guarantees that the
envelope sender address is valid and authorized with the used authentication. This in essence complies with the
SPF model specified in RFC 7208.
* ''May add sender'' permits to add a Sender address header field if the
envelope sender address does not match any author address in the "From" header field. This roughly complies with the
Sender ID
Sender ID is an historic anti- spoofing proposal from the former MARID IETF working group that tried to join Sender Policy Framework (SPF) and Caller ID. Sender ID is defined primarily in Experimental RFC 4406, but there are additional parts in RF ...
model specified in RFC 4406 – ignoring the tricky case of Resent-From header fields not covered in RFC 6409.
See also
*
E-mail authentication Email authentication, or validation, is a collection of techniques aimed at providing verifiable information about the origin of email messages by validating the domain ownership of any message transfer agents (MTA) who participated in transferring ...
*
List of mail servers
This is a list of mail server software: mail transfer agents, mail delivery agents, and other computer software which provide e-mail.
Product statistics
All such figures are necessarily estimates because data about mail server share is diffi ...
*
Comparison of mail servers
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 i ...
*
Smart host 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.
Smarthosts were originally open mail relays, but most providers now requiring authentication fro ...
*
E-mail agent (infrastructure)
An e-mail agent is a program that is part of the e-mail infrastructure, from composition by sender, to transfer across the network, to viewing by recipient. The best-known are message user agents (MUAs, aka, e-mail clients) and message transfer a ...
(MxA)
*
Mail delivery agent A message delivery agent (MDA), or mail delivery agent, is a computer software component that is responsible for the delivery of e-mail messages to a local recipient's mailbox., ''Internet Mail Architecture'', D. Crocker (July 2009) It is also call ...
(MDA)
*
Mail user agent
The mail or post is a system for physically transporting postcards, letters, and parcels. A postal service can be private or public, though many governments place restrictions on private systems. Since the mid-19th century, national postal syst ...
(MUA)
References
*
*
{{Reflist
Email agents
Email authentication
Internet mail protocols