|Network Working Group||C Hutzler|
|INTERNET DRAFT||America Online|
|Category: Best Current Practice||Brandenburg InternetWorking|
|Expires: November 2005||P.W. Resnick|
Email Submission Between Independent Networks
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Copyright © The Internet Society (2005). All Rights Reserved.
Email has become a popular distribution service for a variety of socially unacceptable, mass-effect purposes. The most obvious ones include spam and worms. This note recommends conventions for the operation of email submission and transport services between independent operators, such as enterprises and Internet Service Providers. Its goal is to improve lines of accountability for controlling abusive uses of the Internet mail service. Consequently the document offers recommendations for constructive operational policies between independent operators of email transmission services.
The document seeks BCP status. Comments and discussion of this document should be addressed to the email@example.com mailing list.
The very characteristics that make email such a convenient communications medium -- its near ubiquity, rapid delivery and low cost -- have made it a fertile ground for the distribution of unwanted or malicious content. Spam, fraud and worms have become a serious problem, threatening the viability of email and costing end users and providers millions of dollars in damages and lost productivity. In recent years, independent operators including enterprises and ISPs have turned to a number of different technologies and processes, in an attempt to combat these problems, with varying effect and with vastly different impacts on users and on the Internet mail infrastructure.
Email will often travel between multiple independent providers of email transmission services, en route to its final destination. They will generally have no prior arrangement with one another and may employ different rules on the transmission. It is therefore difficult both to debug problems that occur in mail transmission and to assign accountability if undesired or malicious mail is injected into the Internet mail infrastructure.
This document suggests operational policies that independent operators of email transmission services may adopt, to assist in providing continued, smooth operation of Internet email, but with controls in place to improve accountability. These policies are appropriate for operators of all sizes and may be implemented by operators independently, without regard for whether the other side of an email exchange has implemented them.
It is important to note that the adoption of these policies alone will not solve the problems of spam and other undesirable email. However they provide a useful step in clarifying lines of accountability and interoperability between operators. This will help raise the bar for abusers, and will pave the way for additional tools to preserve the utility of the Internet email infrastructure.
The Internet email architecture distinguishes four message-handling components:
At the origination end, an MUA works on behalf of end users to create a message and perform initial "submission" into the transmission infrastructure, via an MSA. An MSA accepts the message submission, performs any necessary preprocessing on the message and relays the message to an MTA for transmission. MTAs relay messages to other MTAs, in a sequence reaching a destination MDA that, in turn, delivers the email to the recipient's inbox. The inbox is part of the recipient-side MUA that works on behalf of the end-user to process received mail.
These architectural components are often compressed, such as having the same software do MSA, MTA and MDA functions. However the requirements for each of these components of the architecture are becoming more extensive, so that their separation is increasingly common.
Note: The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119].
The MSA, MTA and MDA functions used to be considered as the same set of functions. This has been reflected in the history of Internet mail by having MSA, MTA and MDA transfers all be performed with SMTP [RFC2821] [RFC0821], over TCP Port 25. Internet mail permits email to be exchanged with no prior arrangement. Hence Port 25 exchanges occur without sender authentication. That is, the sender is not necessarily known by the relaying MTAs or the MDA.
It is important to distinguish MUA-to-MSA email submission, versus MTA relaying, versus the final MTA-to-MDA transmission, prior to MDA-to-MUA delivery. Submission typically does entail a relationship between client and server; equally, the MDA can determine that it will be effecting final delivery and has an existing relationship with the recipient. That is, MSAs and MDAs can take advantage of having prior relationships with users, in order to constrain their transfer activities.
Specifically, an MSA can choose to reject all postings from MUAs for which it has no existing relationship. Similarly, an MDA can choose to reject all mail to recipients for which that MDA has no arrangement to perform delivery. Indeed, both of these policies are already in common practice.
Best practices are:
An MUA, such as one desiring enforced privacy, may need to submit mail across the Internet, rather than to a local MSA. This requirement creates a challenge for the provider operating the network that hosts the MUA. It makes that provider an involuntary recruit to the task of solving mass-effect email problems. When the MUA participates in a problem that affects large numbers of Internet users, the operator is expected to effect remedies and is often expected to prevent such occurrences.
A proactive technique used by some providers is to block all outbound Port 25 SMTP traffic or to automatically redirect this traffic through a local SMTP proxy, except for hosts that are explicitly authorized. This can be problematic for some users, notably legitimate mobile users attempting use their "home" MSA, even though those users might already employ legitimate, Port 25-based authentication.
This document offers no recommendation concerning the blocking of SMTP Port 25.
Rather, it pursues the constructive benefit of using the official SUBMISSION Port 587 [RFC2476].
Best practices are:
Note that the requirement for authentication, on the part of the MSA, thereby makes that MSA responsible for the ensuing traffic it generates.
Figure 1 depicts a local user (MUA.l) submitting a message to an MSA (MSA). It also shows a remote user (MUA.r), such as might be in a coffee shop offering "hotspot" wireless access, submitting a message to their "home" MSA via an Authenticated Port 587 transaction.
"Home" Network SMTP /--------\ Destination +-------+ +-----+ port 25 | | +----------+ | MUA.l | -> | MSA | ------> | | -> | MDA | +-------+ 25 +-----+ | INTERNET | 25 +----------+ or ^ | | 587 \--------<---|---\ | \---\----/ ^ SUBMISSION | Port 587 +--------+ | MUA.r | +--------+ "HotSpot"
Figure 1: Example of Port 587 Usage
There are many different technologies available to authenticate message submission transactions. These range from simple mechanisms like POP authorization before SMTP and IP Address access lists, all the way to SMTP AUTH [RFC2554] and client side certificates using Transport Layer Security (TLS) [RFC3207]. Depending on the environment, each of these mechanisms can be more or less effective and convenient. Organizations are encouraged to choose the most secure approach that is practical.
For example, SMTP AUTH using a secure authentication method like CRAM-MD5 or DIGEST-MD5 may be sufficient. However, in some environments, it is impractical to use one of the secure methods, meaning that SMTP AUTH would be transmitting the username and the password in clear text over insecure networks. This could allow attackers to listen for this traffic and steal account data. In these cases, using STARTTLS to establish an encrypted channel for transmission of the SMTP AUTH username and password would be preferred. Similarly, STARTTLS with client side certificates could be used with the SMTP AUTH EXTERNAL mechanism to achieve secure authentication.
Email transfer between independent administrations can be the source of large volumes of unwanted email and email containing malicious content designed to attack the recipient's system. This document addresses the requirements to permit such exchanges while reducing the likelihood that malicious mail will be transmitted.
|[RFC0821]||Postel, J.B., “Simple Mail Transfer Protocol”, STD 10, RFC 821, August 1982.|
|[RFC2476]||Gellens, R. and J.C. Klensin, “Message Submission”, RFC 2476, December 1998.|
|[RFC2821]||Klensin, J., “Simple Mail Transfer Protocol”, RFC 2821, April 2001.|
|[RFC2119]||Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels”, BCP 14, RFC 2119, March 1997.|
|[RFC2554]||Myers, J.G., “SMTP Service Extension for Authentication”, RFC 2554, March 1999.|
|[RFC3207]||Hoffman, P., “SMTP Service Extension for Secure SMTP over Transport Layer Security”, RFC 3207, February 2002.|
These recommendations were first formulated during informal discussions among members of Anti-Spam Technical Alliance (ASTA) and some participants from the Internet Research Task Force's Anti-Spam Research Group (ASRG).
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