RFC 5068
Email Submission Operations: Access and Accountability Requirements
RFC 5068: Email Submission Operations: Access and Accountability Requirements 💌
Okay bestie, this one is all about the very first step of sending an email, the part where your message gets handed off into the mail system. 🦋
Here's the title block tea ☕. This is a Best Current Practice (BCP 134), published in November 2007 in the Network Working Group. The authors are C. Hutzler, D. Crocker (Brandenburg InternetWorking), P. Resnick (QUALCOMM Incorporated), E. Allman (Sendmail, Inc.), and T. Finch (University of Cambridge Computing Service). There's no Obsoletes or Updates line in its title block, so it doesn't replace or change any other RFC. 🌸
Status of This Memo 📜
This doc is a Best Current Practice for the Internet Community, and it's literally asking for discussion and suggestions for improvements. Distribution of this memo is unlimited. ✨
Abstract 💭
Email has become a popular distribution service for all kinds of socially unacceptable, mass-effect purposes, and the most obvious ones are spam and worms. 🙈 This note recommends conventions for how email submission and transport services should run between independent operators, like enterprises and Internet Service Providers. The goal is to improve lines of accountability for controlling abusive uses of the Internet mail service. So this document offers recommendations for constructive operational policies between independent operators of email submission and transmission services. 💅
Email authentication technologies are all about providing assurances and traceability between internetworked networks. In a lot of email services, the weakest link in the chain of assurances is the initial submission of a message. This document gives recommendations for constructive operational policies for this first step of email sending, the submission (or posting) of email into the transmission network. Relaying and delivery involve policies that happen after submission, so they're outside the scope of this document. 🫶
Table of Contents 🗂️
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Submission, Relaying, Delivery . . . . . . . . . . . . . . . . 4
3.1. Best Practices for Submission Operation . . . . . . . . . 5
3.2. Transitioning to Submission Port . . . . . . . . . . . . . 6
4. External Submission . . . . . . . . . . . . . . . . . . . . . 6
4.1. Best Practices for Support of External Submissions . . . . 7
5. Message Submission Authentication/Authorization
Technologies . . . . . . . . . . . . . . . . . . . . . . . . . 8
6. Security Considerations . . . . . . . . . . . . . . . . . . . 9
7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 9
7.1. Normative References . . . . . . . . . . . . . . . . . . . 9
7.2. Informative References . . . . . . . . . . . . . . . . . . 9
Appendix A. Acknowledgments . . . . . . . . . . . . . . . . . . . 101. Introduction 🌷
Okay so the exact things that make email so convenient are the same things that make it a playground for unwanted or malicious content. We're talking near ubiquity, rapid delivery, low cost, and support for exchanges without any prior arrangement. 😭 Spam, fraud, and worms have become a serious problem, costing end users and providers millions of dollars in damages and lost productivity. In recent years, independent operators (enterprises and ISPs included) have tried a bunch of different technologies and procedures to fight back. The results have been mixed, at best. 💀
On its way to its final destination, email will often travel between multiple independent providers of email transmission services. These services generally have no prior arrangement with one another, and they may use different rules on transmission. So it's genuinely hard both to debug problems that happen in mail transmission and to assign accountability if undesired or malicious mail gets injected into the Internet mail infrastructure. 🕵️♀️
Many email authentication technologies exist, and they provide some accountability and traceability between disparate networks. This document builds on those technologies by exploring best practices for authenticating and authorizing the first step of an email's delivery: from a Mail User Agent (MUA) to a Mail Submission Agent (MSA), known as submission. Without strong practices on email submission, the authentication technologies used elsewhere in the service provide limited benefit. 🎀
This document specifies operational policies for that first step of email sending, the submission (posting from an MUA to an MSA, as defined below) of email into the transmission service. These policies will keep Internet email running smoothly while adding controls that improve accountability. Relaying and delivering use policies that happen after submission, so they're outside the scope of this document. The policies here are appropriate for operators of all sizes of networks, and operators can implement them independently, without regard for whether the other side of an email exchange has implemented them. 🧚
It's important to note that adopting these policies alone won't solve the problems of spam and other undesirable email. 😤 But they're a useful step in clarifying lines of accountability and interoperability between operators. This helps raise the bar against abusers and provides a foundation for additional tools that preserve the utility of the Internet email infrastructure. 👑
NOTE: This document does not delve into other anti-spam operational issues such as standards for rejection of email. The authors note that this could be a valuable effort to undertake and encourage its pursuit.
2. Terminology 📖
The Internet email architecture distinguishes four message-handling components:
- Mail User Agents (MUAs)
- Mail Submission Agents (MSAs)
- Mail Transfer Agents (MTAs)
- Mail Delivery Agents (MDAs)
At the origination end, an MUA works on behalf of end users to create a message and perform the 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 that reaches a destination MDA, which then 'delivers' the email to the recipient's inbox. The inbox is part of the recipient-side MUA, which works on behalf of the end user to process received mail. 📬
These architectural components are often squished together, like having the same software do MSA, MTA, and MDA functions. 🫠 However, the requirements for each of these components are becoming more extensive, so their software and even their physical platforms are increasingly separated. 💜
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]. 🔑
3. Submission, Relaying, Delivery 🚀
Originally, the MSA, MTA, and MDA architectural components were considered a single unit. This was reflected in the practice of having MSA, MTA, and MDA transfers all performed with SMTP [RFC2821] [RFC0821], over TCP port 25. Internet mail lets email be exchanged without prior arrangement and without sender authentication. That means the confirmed identity of the message's originator is not necessarily known by the relaying MTAs or the MDA. 😳
It's really important to tell apart MUA-to-MSA email submission, MTA relaying, and the final MTA-to-MDA transition. Submission typically does involve a pre-established relationship between the user of the client and the operator of the server. Meanwhile, the MDA is doing the final delivery and can determine whether it has an existing relationship with the recipient. So MSAs and MDAs can use their prior relationships with users to constrain their transfer activities. 🥹
Specifically, an MSA can choose to reject all postings from MUAs it has no existing relationship with. Similarly, an MDA can choose to reject all mail to recipients it has no arrangement to deliver to. Honestly, both of these policies are already common practice. 🙌
3.1. Best Practices for Submission Operation 🛠️
Submission Port Availability:
If external submissions are supported (that is, from outside a site's administrative domain), then the domain's MSAs MUST support the SUBMISSION port 587 [RFC4409]. Operators MAY standardize on the SUBMISSION port for both external AND LOCAL users, and this can significantly simplify submission operations. ✨
Submission Port Use:
MUAs SHOULD use the SUBMISSION port for message submission. 💖
Submission Authentication:
MSAs MUST perform authentication on the identity asserted during all mail transactions on the SUBMISSION port, even for a message whose RCPT TO address wouldn't cause the message to be relayed outside the local administrative domain. 🔐
Submission Authorization:
An operator of an MSA MUST ensure that the authenticated identity is authorized to submit email, based on an existing relationship between the submitting entity and the operator. This requirement applies to all mail submission mechanisms (MUA to MSA). 🪪
Submission Accountability after Submission:
For a reasonable period of time after submission, the message SHOULD be traceable by the MSA operator to the authenticated identity of the user who sent it. Such tracing MAY be based on transactional identifiers stored in the headers (received lines, etc.) or other fields in the message, on audit data stored elsewhere, or on any other mechanism that supports sufficient post-submission accountability. The specific length of time after message submission that traceability is supported is not specified here. However, issues regarding transit often occur as much as one week after submission. 🗓️
Note that [RFC3848] defines a means of recording submission-time information in Received header fields. This information can help receive-side analysis software establish a sending MSA's accountability and then make decisions about processing the message. 🌈
3.2. Transitioning to Submission Port 🎀
okay bestie, this is the glow-up era for mail 🌸 so the goal is moving initial message submission off port 25 and over to port 587 🦋
MSAs MUST listen on port 587 by default and they SHOULD have the ability to listen on other ports too ✨ MSAs MUST require authentication on port 587 and they SHOULD require authentication on any other port used for submission 🔐 MSAs MAY also listen on other ports, no cap 💅 but no matter which ports messages come in on, MSAs MUST NOT permit relaying of unauthenticated messages to other domains. That means they must not be open relays 🙅♀️ like, absolutely not, that's a hard no 💀
now the MUA side 🧚 MUAs SHOULD attempt to find the best possible submission port from a list of alternatives as a default. And the SUBMISSION port 587 SHOULD be placed first in the list 👑 Here's the tea though: most MUAs out there today do not permit falling back to alternate ports, so sites SHOULD pre-configure or encourage their users to connect on the SUBMISSION port 587, assuming that site supports that port 🫶 it's giving organized, it's giving intentional 🌈
4. External Submission 🦋
okay so picture this: an MUA might need to submit mail across the Internet instead of to a local MSA 🌍 why? bc it wants special services from its home site 💜 like active privacy protection against third-party content monitoring, timely processing, and being subject to the most appropriate authentication and accountability protocols ✨ and the privacy thing can reasonably include protection against monitoring by the operator of the MUA's access network too 👀
this creates a bit of a dilemma for the provider running the IP network the MUA uses to get online 😭 they become an involuntary recruit for solving mass-effect email problems!! like, when an MUA is part of a problem hitting tons of Internet users, that provider is expected to fix it and is often expected to prevent it from happening in the first place 🙈 not fair, but it's the reality bestie 💅
a proactive move some providers make is to block all outbound use of port 25 SMTP for mail, or to automatically redirect that traffic through a local SMTP proxy, except for hosts that are explicitly authorized 🚧 lowkey problematic for some users though, especially legit mobile users trying to use their "home" MSA, even when those users might already use legitimate, port 25-based authentication 🥺
This document offers no recommendation concerning the blocking of SMTP port 25 or similar practices for controlling abuse of the standard anonymous mail transfer port. Rather, it pursues the mutually constructive benefit of using the official SUBMISSION port 587 [RFC4409].
(so no hot takes from this doc on blocking port 25 🙅♀️ it just wants everyone vibing on the official SUBMISSION port 587 [RFC4409] instead 🎀)
NOTE: 📝 Many established practices for controlling abuse of port 25, for mail that is being sent outbound, currently do exist 💖 these include proxying SMTP traffic to local hosts for screening, combined with various forms of rate limits. The authors suggest that a separate document on this topic would benefit the email operations community 🌸
4.1. Best Practices for Support of External Submissions 💅
Open Submission Port:
Access Providers MUST NOT block users from accessing the external Internet using the SUBMISSION port 587 [RFC4409] 🙌
Traffic Identification -- External Posting (MSA) Versus Relaying (MX):
When email service providers receive email from outside their local operational environment, they MUST distinguish between unauthenticated email addressed to local domains (MX traffic) versus submission-related authenticated email that can be addressed anywhere (MSA traffic). 🕵️♀️ this lets the MTA restrict relaying operations, and that's how it prevents "open" relays 🔒 note that there are situations where this may not apply, like secondary MXs and related implementations internal to an operator's network and within their control 🧚
Figure 1 is giving tour guide energy 🗺️ it shows a local user (MUA.l) submitting a message to an MSA (MSA). it also shows a remote user (MUA.r), like someone in a coffee shop with "hotspot" wifi, submitting to their "home" MSA through an authenticated port 587 transaction ☕ the figure also shows using port 587 or port 25 inside the MSA's network. This document makes no recommendations about the use of port 25 for submission. the diagram just notes that port 25 is in common use and acknowledges it can be used with sufficient accountability within an organization's network 🌈
HOME NETWORK DESTINATION
+-------+
| MUA.l |
+---+---+
port | port port port
587/25 V 25 25 -------- 25
+-----+ +-----+ ****** / \ ****** +-----+ +-----+
| MSA |->| MTA |->* AP *->| |->* AP *->| MTA |->| MDA |
+--^--+ +-----+ ****** | INTERNET | ****** +-----+ +-----+
| | |
+-------<--------------|----+ |
\ | /
---^----
|
******
AP = Access Provider * AP *
******
| port 587
+---+----+
| MUA.r |
+--------+
HOTSPOTFigure 1: Example of Port 587 Usage via Internet ✨
5. Message Submission Authentication/Authorization Technologies 🔐
there are loads of competent technologies and standards for authenticating message submissions 💖 two component mechanisms that have been standardized are SMTP AUTH [RFC4954] and TLS [RFC3207]. depending on the environment, different mechanisms can be more or less effective and convenient, and they might have to be combined with each other to make a secure system 🧩 Organizations SHOULD choose the most secure approaches that are practical. 💅
this doc doesn't give recommendations on specific security implementations, it just issues a warning 🚨 transmitting user credentials in clear text over insecure networks SHOULD be avoided in all scenarios, because attackers could listen for this traffic and steal account data 😱 in these cases it is strongly suggested that an appropriate security technology MUST be used. 🛡️
6. Security Considerations 💀
email transfer between independent administrations can be the source of huge volumes of unwanted email, plus email with malicious content designed to attack the recipient's system 😭 this document addresses the requirements and procedures that allow these exchanges while reducing the likelihood that malicious mail gets transmitted 🫶
7. References 📚
7.1. Normative References 💜
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2821] Klensin, J., "Simple Mail Transfer Protocol", RFC 2821,
April 2001.
[RFC4409] Gellens, R. and J. Klensin, "Message Submission for Mail",
RFC 4409, April 2006.7.2. Informative References 🌸
[RFC0821] Postel, J., "Simple Mail Transfer Protocol", STD 10,
RFC 821, August 1982.
[RFC3207] Hoffman, P., "SMTP Service Extension for Secure SMTP over
Transport Layer Security", RFC 3207, February 2002.
[RFC3848] Newman, C., "ESMTP and LMTP Transmission Types
Registration", RFC 3848, July 2005.
[RFC4954] Siemborski, R., Ed. and A. Melnikov, Ed., "SMTP Service
Extension for Authentication", RFC 4954, July 2007.Appendix A. Acknowledgments 🙏
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) 🌟
later reviews and suggestions were provided by: M. Allman, L.H. Aestrand, N. Borenstein, S. Bortzmeyer, K. Chon, R. Clayton, B. Cole, W. Dnes, V. Duchovni, E. Edelstein, F. Ellermann, M. Elvey, J.D. Falk, N. Freed, J. Glube, A. Herzberg, J. Klensin, J. Levine, S. Moonesamy, K. Moore, R. Nelson, C. Newman, C. O'Malley, S. Ramasubramanian, R. Rognlie, J. St. Sauver, W. Schlitt, B. Shein, B. Sullivan. 🫶
Authors' Addresses 📮
Carl Hutzler
2512 Freetown Drive
Reston, VA 20191
Phone: 703-915-6862
EMail: cdhutzler@aol.com
URI: http://carlhutzler.com/blog/
Dave Crocker
Brandenburg InternetWorking
675 Spruce Dr.
Sunnyvale, CA 94086
USA
Phone: +1.408.246.8253
EMail: dcrocker@bbiw.net
URI: http://bbiw.net
Peter Resnick
QUALCOMM Incorporated
5775 Morehouse Drive
San Diego, CA 92121-1714
USA
Phone: +1 858 651 4478
EMail: presnick@qualcomm.com
URI: http://www.qualcomm.com/~presnick/
Eric Allman
Sendmail, Inc.
6745 Christie Ave., Suite 350
Emeryville, CA
USA
Phone: +1 510 594 5501
EMail: eric+ietf-smtp@sendmail.org
Tony Finch
University of Cambridge Computing Service
New Museums Site
Pembroke Street
Cambridge CB2 3QH
ENGLAND
Phone: +44 797 040 1426
EMail: dot@dotat.at
URI: http://dotat.at/Full Copyright Statement 🌈
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- ietf: rfc5068 2007-11
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