Structured Email
draft-ietf-sml-structured-email-06
| Document | Type | Active Internet-Draft (sml WG) | |
|---|---|---|---|
| Author | Hans-Jörg Happel | ||
| Last updated | 2026-06-01 | ||
| Replaces | draft-happel-structured-email | ||
| RFC stream | Internet Engineering Task Force (IETF) | ||
| Intended RFC status | Proposed Standard | ||
| Formats | |||
| Additional resources | Mailing list discussion | ||
| Stream | WG state | WG Document | |
| Document shepherd | Arnt Gulbrandsen | ||
| IESG | IESG state | I-D Exists | |
| Consensus boilerplate | Yes | ||
| Telechat date | (None) | ||
| Responsible AD | (None) | ||
| Send notices to | arnt@gulbrandsen.priv.no |
draft-ietf-sml-structured-email-06
SML H.-J. Happel
Internet-Draft audriga GmbH
Intended status: Standards Track 1 June 2026
Expires: 3 December 2026
Structured Email
draft-ietf-sml-structured-email-06
Abstract
This document specifies how a machine-readable variant of its content
can be added to email messages.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on 3 December 2026.
Copyright Notice
Copyright (c) 2026 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents (https://trustee.ietf.org/
license-info) in effect on the date of publication of this document.
Please review these documents carefully, as they describe your rights
and restrictions with respect to this document. Code Components
extracted from this document must include Revised BSD License text as
described in Section 4.e of the Trust Legal Provisions and are
provided without warranty as described in the Revised BSD License.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Conventions Used in This Document . . . . . . . . . . . . . . 3
3. Representing structured data . . . . . . . . . . . . . . . . 3
3.1. Knowledge representation language . . . . . . . . . . . . 4
3.2. Vocabularies . . . . . . . . . . . . . . . . . . . . . . 4
4. Structured data in email messages . . . . . . . . . . . . . . 5
4.1. Content-types . . . . . . . . . . . . . . . . . . . . . . 5
4.2. Designation . . . . . . . . . . . . . . . . . . . . . . . 5
4.3. Placement . . . . . . . . . . . . . . . . . . . . . . . . 6
4.3.1. Full representation . . . . . . . . . . . . . . . . . 6
4.3.2. Partial representation . . . . . . . . . . . . . . . 7
4.3.3. Non-representation . . . . . . . . . . . . . . . . . 7
4.4. Identifiers . . . . . . . . . . . . . . . . . . . . . . . 8
4.4.1. Using identifiers in structured data . . . . . . . . 8
4.4.2. Using structured data identifiers in text/html . . . 8
5. Structured data across email messages . . . . . . . . . . . . 9
5.1. Forwarding . . . . . . . . . . . . . . . . . . . . . . . 9
5.2. Replies . . . . . . . . . . . . . . . . . . . . . . . . . 9
5.3. Updates . . . . . . . . . . . . . . . . . . . . . . . . . 10
6. Message flags . . . . . . . . . . . . . . . . . . . . . . . . 10
7. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 11
7.1. Full representation . . . . . . . . . . . . . . . . . . . 11
7.2. Full representation (machine-readable only) . . . . . . . 11
7.3. Full representation with inline image . . . . . . . . . . 11
7.4. Partial representation . . . . . . . . . . . . . . . . . 11
7.5. Non-representation . . . . . . . . . . . . . . . . . . . 11
8. Appendix (Structured Email Signature) . . . . . . . . . . . . 12
9. Security and trust . . . . . . . . . . . . . . . . . . . . . 12
10. Implementation status . . . . . . . . . . . . . . . . . . . . 12
10.1. Structured Email for Nextcloud Mail . . . . . . . . . . 13
10.2. Structured Email plugin for Roundcube Webmail . . . . . 13
10.3. Yatagarasu Mail . . . . . . . . . . . . . . . . . . . . 13
11. Security considerations . . . . . . . . . . . . . . . . . . . 13
12. Privacy considerations . . . . . . . . . . . . . . . . . . . 14
13. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 14
13.1. Creation of the SML registry group . . . . . . . . . . . 14
13.1.1. Creation of the SML Email Vocabulary Registry . . . 14
13.1.2. Creation of the SML Vocabularies Registry . . . . . 14
13.1.3. Initial Entries for the SML Vocabularies Registry . 14
13.2. Registration of the Content-Purpose MIME header field . 15
13.3. Registration of the $hasStructuredData keyword . . . . . 15
13.4. Registration of the $MRM keyword . . . . . . . . . . . . 16
14. Informative References . . . . . . . . . . . . . . . . . . . 17
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 19
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1. Introduction
Information on websites and in email messages mostly targets human
readers. However, various attempts have been made to make such
information – (U+2013) fully or in part – (U+2013) machine-readable,
so that tools can assist users in dealing with that information more
efficiently.
One widespread approach is the usage of [SchemaOrg] vocabulary, which
can be embedded in the HTML markup of websites. Tools such as web
search engines may crawl this information and use it for improving
the quality of search result snippets (e.g., by showing ratings,
opening hours, or contact information).
Similarly, some web shops, hotels, and airlines include Schema.org
vocabulary in their order receipt email messages sent to customers.
This information is extracted and used by some ISPs and open source
tools ([SchemaOrgEmail]).
However, these implementations differ in many details. The goal of
this specification is to provide a clear and comprehensive
specification for this practice and to provide ground for potential
future extensions.
2. Conventions Used in This Document
The terms "message" and "email message" refer to "electronic mail
messages" or "emails" as specified in [RFC5322]. The term "Message
User Agent" (MUA) denotes an email client application as per
[RFC5598].
The terms "machine-readable data" and "structured data" are used in
contrast to "human-readable" messages and denote information
expressed "in a structured format (..) which can be consumed by
another program using consistent processing logic" [MachineReadable].
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP
14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
3. Representing structured data
In order to exchange structured data, one needs to choose a formal
language and a serialization format. Based on this, vocabularies can
be used to establish a shared understanding of structured data among
different parties, such as email senders and receivers.
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3.1. Knowledge representation language
The Resource Description Framework ([RDF]) is a formal language for
knowledge representation standardized by the W3C. It is underlying
[SchemaOrg] and thus already used for annotating websites and emails.
Among the various serializations for RDF, JSON-LD ([JSONLD]) has
become the most commonly used serialization used on websites
([WDCStats]).
Hence, structured data in email messages SHOULD be expressed in the
JSON-LD serialization of RDF.
For discussion, see also:
https://github.com/hhappel/draft-happel-structured-email/issues/1
3.2. Vocabularies
Using RDF/JSON-LD, users are free to express any kind of information
in structured data. For reuse and reference however, it is common to
agree upon core concepts/entities and properties for a certain
domain. Those are typically collected and shared in so-called
vocabularies.
[SchemaOrg] is a widespread vocabulary, which was initially designed
by search engine vendors for annotating content on websites. A small
subset of its concepts is already used by email senders and processed
by email providers.
Users that want to add structured data into email message SHOULD use
concepts from [SchemaOrg], if they fit their use case. They MAY
however use any valid JSON-LD.
There might also be certain vocabularies for email-specific use cases
(such as [I-D.ietf-sml-structured-vacation-notices-01]), that will be
specifically endorsed by the IETF or by respective RFCs.
MUAs may choose freely (in consent with its users) if and how to use
structured data extracted from messages. If they do not explicitly
support a certain vocabulary, MUAs may also rely on extensions or
passing data to outside applications, similar to the case of "email
attachments" (i.e., MIME body parts with content-disposition type
attachment [RFC2183]).
For discussion, see also:
https://github.com/hhappel/draft-happel-structured-email/issues/2
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4. Structured data in email messages
This section defines aspects of adding structured data to a MIME
message and its interrelation with other body parts.
As a basic distinction, we define two types of messages:
* Machine-readable messages (MRM), for which their content is
entirely machine-readable
* Partly machine-readable messages (PMRM), for which their content
is NOT entirely machine-readable (e.g., only describing some of
the human-readable content)
4.1. Content-types
Structured data MUST be added to a MIME message as an application/
ld+json body part, which is meant to provide a machine-readable
description of the human-readable content of a message part.
Alternatively, senders can use a content-type from which application/
ld+json can be easily derived, such as application/jose ([RFC7515]).
For backwards-compatibility, MUAs MAY choose to consider parsing
machine-readable content from text/html body parts, as used by some
legacy structured email senders ([SchemaOrgEmail]). Structured data
can be embedded as JSON-LD _or_ Microdata ([Microdata]) in those
emails. MUAs are free to parse either syntax. However, MUA and
other senders MUST NOT use one of these legacy formats when _sending_
structured email.
4.2. Designation
This document targets structured data describing the content of an
email message itself or complementary structured data to inform the
MUA (see "non-representation" case below).
This is opposed to structured data which is merely added to an email
message as a regular MIME body part for other purposes (i.e., as a
normal file attachment).
To distinguish these two cases, senders MUST set a header field
Content-Purpose to the value Machine-readable on a body part, which
is meant to provide machine-readable content. A MUA SHOULD NOT show
such body parts as a file attachment in the list of email
attachments.
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4.3. Placement
When aiming to describe human-readable content in a machine-readable
way, there may exist three general relations between both types of
content. In these three cases, the machine-readable version of the
content is either:
* fully representative of the human-readable content ("Full
representation")
* describing only parts of the human-readable content ("Partial
representation")
* describing none of the human-readable content ("Non-
representation")
Those distinctions matter for MUAs, as they can make choices for the
autoprocessing or presentation of messages and their body parts.
For discussion, see also:
https://github.com/hhappel/draft-happel-structured-email/issues/3
4.3.1. Full representation
Full representation denotes the case, in which structured data
describes the content of a certain body part in the sense of an
"_alternative_ version of the same information" as in the informal
definition of multipart-alternative in [RFC2046].
If a message is sent to a human recipient, a sender MUST use a
multipart/alternative for each body part that is _fully_ described by
structured data. In this case, the multipart/alternative should
contain a text/plain and a text/html version of the content for
backwards compatibility, plus the application/ld+json body part
containing the structured data representation.
If the MIME tree of a message consists entirely of body parts which
have a "full representation" machine-readable alternative of their
content, this is called a "machine-readable message" (MRM). A MRM
may be suitable for special forms of automated processing, since it
contains no information for which interpretation of a human reader is
required.
While it may be tricky to derive if a message is a MRM in case of
complex MIME trees, the majority of messages in practice will contain
just one multipart/alternative body part, for which such conclusion
is easy to derive.
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If a MUA is able to process the vocabulary of a MRM or is able to
process the structured data otherwise, it SHOULD prefer the
application/ld+json representation, unless instructed otherwise by
the user. In case of more complex MIME structures, it is up to the
discretion of the MUA how to process or render the message.
If a sender intends to send a MRM, the _subject_ of a message MUST
NOT contain any additional information which is not contained in the
machine-readable parts of the message. This is to ensure that MRM
may be processed fully automatically if desired.
Some countries require senders to include legal disclaimers in email
messages. In the case of "full representation", a sender MAY include
a "structured email signature" as shown in the Appendix either in the
"full representation" structure data or in an additional "non-
representation" body part.
If a message is sent to a definitely non-human recipient (e.g., an
API), human-readable content MAY be omitted from the message entirely
and application/ld+json MAY be used as Content-Type in the message
header.
4.3.2. Partial representation
If structured data is intended to describe only a _subset_ of a
certain human-readable body part, it MUST be added as a multipart/
related entity.
This multipart/related entity MUST also contain the human textual
content of the body part (e.g., text/plain and text/html). Also, any
MIME body part referenced from the structured data in the structured
data body part MUST be enclosed in this multipart/related entity.
MUAs SHOULD render such messages as if no structured data body part
would be included. MUAs MAY process the structured data for
providing an enhanced user experience of their resp. the user's
choice.
4.3.3. Non-representation
In the case of non-representation, there is no overlap between
structured data and the human readable content.
This may be useful for special scenarios, such as embedding
"preemptive" structured vacation notices as described in [I-D.happel-
sml-structured-vacation-notices-00] into email messages.
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As in the case of partial representation, MUAs receiving such
messages may take according action based on the structured data
extracted.
4.4. Identifiers
There are existing use cases for cross-referencing between different
parts of a MIME message, embedded images probably being the most
widespread one. For this purpose, [RFC2392] defines the cid: and
mid: URI schemes.
In a similar fashion, cross-referencing might occur between
structured data and other message parts.
4.4.1. Using identifiers in structured data
Most nodes and properties in JSON-LD are identified using IRIs
[RFC3987]. Since any [RFC2392] (cid:/mid:) reference forms a valid
IRI, those references can be directly used in JSON-LD.
There are two main cases for which cid:-identifiers SHOULD be used in
structured data.
First, if structured data references binary content such as images or
other files, which already exist as MIME body parts within the same
message.
Second, if a cid: value is used in a JSON-LD @id property, the
corresponding JSON-LD node can be considered to describe the MIME
body part identified by that cid:. This MAY be used to denote that
certain structured data is explicitly describing that MIME body part,
such as an image. This particularly MUST NOT be used for the main
text/plain or text/html body parts, though.
For discussion, see also:
https://github.com/hhappel/draft-happel-structured-email/issues/4
4.4.2. Using structured data identifiers in text/html
In the case of "partial representation", a MUA should still primarily
display the human readable part of a message (e.g., text/plain or
text/html).
It might however be helpful if the MUA is able to determine which
parts of human readable text refer to certain structured data - e.g.,
to offer actions based on structured data directly in the context of
the corresponding human-readable content.
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For this purpose, the sender may add a HTML "data-id" property
([HTMLData]) to any HTML entity in the text/html body, which
references the @id property of a JSON-LD node in the structured data.
Besides referencing the corresponding JSON-LD node, a sender might
also want to denote if the underlying data is "extensively described"
or just "mentioned" in the human readable representation. For
example, the New York Times cooking newsletter typically _features_
few recipes, while _mentioning_ a larger number of further recipes,
also referencing their web URL.
For providing an adequate user experience, the MUA should be able to
understand which recipes are featured in an email and which are just
mentioned.
For discussion, see also:
https://github.com/hhappel/draft-happel-structured-email/issues/5
5. Structured data across email messages
This sections deals with aspects that go beyond the scope of an
individual MIME message.
5.1. Forwarding
Forwarding messages including structured data needs to be considered
from a privacy perspective, particularly in cases of "non-
representation", when the user has no way to determine structured
data from the human readable part of the message.
A MUA MUST strip non-representative structured data when a user is
forwarding messages to somebody else in her MUA. Note that this does
not apply to automated forwarding of messages.
Beyond that, privacy issues also apply to forwarding regular email
messages. Improvements of the status quo might hence be considered
beyond the specific context of structured email.
For discussion, see also:
https://github.com/hhappel/draft-happel-structured-email/issues/6
5.2. Replies
Users or agents might want to reply to structured emails with a
structured email. Details for this will be specified in a separate
future specification.
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5.3. Updates
In human-readable messages, human language can be used to update or
recall information that was conveyed in prior messages. Accordingly,
there needs to be a machine-readable mechanism that allows to express
the update or recall of structured data.
To update or recall structured data, senders MUST set the SUPERSEDES
header field ([RFC4021]) of the "update" message with the message id
of the original email message. An "update" message with empty
structured data can be used to signal a full recall of previously
sent structured data. Every "update" message MUST have its own
unique message id.
The processing of an "update" message by the receiving MUA is up to
its own discretion, as meaningful action may depend an multiple
factors.
MUAs MAY consider:
* An update might be triggered by a previous action of the user
(e.g., canceling a reservation)
* Adding the original message id as an identifier property to
capture its origin when persisting structured data
For discussion, see also:
https://github.com/hhappel/draft-happel-structured-email/issues/9
6. Message flags
In some use cases, MUAs might benefit from information about message
details without having to evaluate the full message body.
For example, the $hasAttachment IMAP flag ([HasAttachment]) was
proposed to signal the existence of MIME attachments in a message
which otherwise would need to be redetermined based on complex MIME
parsing.
In case of structured data, the receiving MTA (or any later MUA) MAY
determine two facts when parsing the MIME tree of a message:
* If the message contains any structured data according to this
document
* If so, if the message is a fully machine-readable message (MRM)
Accordingly, the following IMAP flags MAY be set for a message:
* $hasStructuredData (in the first case)
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* $MRM (in the second case)
The first case may be helpful for filtering or client-side preloading
of message content. The second case may be particularly helpful for
automated processing without user interaction.
For discussion, see also:
https://github.com/hhappel/draft-happel-structured-email/issues/10
7. Examples
The following section shows some example MIME hierarchies of email
messages containing structured data.
7.1. Full representation
multpart/alternative/
├─ text/plain
├─ text/html
└─ application/ld+json
7.2. Full representation (machine-readable only)
application/ld+json
7.3. Full representation with inline image
multpart/alternative/
├─ text/plain
└─ multipart/related/
├─ multpart/alternative/
│ ├─ text/html
│ ├─ application/ld+json
└─ image/png
7.4. Partial representation
multpart/related/
├─ multipart/alternative/
│ ├─ text/plain
│ └─ text/html
└─ application/ld+json
7.5. Non-representation
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multpart/mixed/
├─ multipart/alternative/
│ ├─ text/plain
│ └─ text/html
└─ application/ld+json
8. Appendix (Structured Email Signature)
The following snippet of structured data uses the Schema.org
publisher property of an EmailMessage.
{
"@context": "https://schema.org/",
"@type": "EmailMessage",
"publisher": {
"@type": "Organization",
"legalName": "MUSEO NACIONAL DEL PRADO DIFUSIÓN, S.A.U., S.M.E.",
"legalAddress": {
"@type": "PostalAddress",
"addressLocality": "Madrid, Spain",
"postalCode": "28014",
"streetAddress": "Casado del Alisal, 10, bajo B"
},
"legalRepresentative" : {
"@context": "https://schema.org",
"@type": "Person",
"name": "Jane Doe",
},
"identifier": {
"@type": "PropertyValue",
"name": "Registration data in the Company Register",
"value": "Volume 23578, Entry 1, Section 8, Sheet M-423094, 74 Folio 74"
},
"vatID": "A84888056"
}
}
9. Security and trust
Email user agents that want to support structured email should follow
guidance to ensure trust and security standards. These will be
elaborated in a separate specification (see [I-D.draft-happel-
structured-email-trust-04]).
10. Implementation status
< RFC Editor: before publication please remove this section and the
reference to [RFC7942] >
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This section records the status of known implementations of the
protocol defined by this specification at the time of posting of this
Internet-Draft, and is based on a proposal described in [RFC7942].
The description of implementations in this section is intended to
assist the IETF in its decision processes in progressing drafts to
RFCs. Please note that the listing of any individual implementation
here does not imply endorsement by the IETF. Furthermore, no effort
has been spent to verify the information presented here that was
supplied by IETF contributors. This is not intended as, and must not
be construed to be, a catalog of available implementations or their
features. Readers are advised to note that other implementations may
exist.
According to [RFC7942], "this will allow reviewers and working groups
to assign due consideration to documents that have the benefit of
running code, which may serve as evidence of valuable experimentation
and feedback that have made the implemented protocols more mature.
It is up to the individual working groups to use this information as
they see fit".
10.1. Structured Email for Nextcloud Mail
Nextcloud Mail is an open source Webmail app which includes a subset
of structured email support since 2020 [NC-Itinerary].
In recent months, code has evolved to support structured email in a
more general fashion [NC-SML].
10.2. Structured Email plugin for Roundcube Webmail
An open source plugin for the Roundcube Webmail software is developed
to serve as an example implementation for this specification
([RC-SML]).
Beyond that, some ISPs and open source tools provide implementation
partly compliant with this specficiation ([SchemaOrgEmail]).
10.3. Yatagarasu Mail
[Yatagarasu] Mail is a fork of the open source Thunderbird Mobile
client for Android. It adds structured email support for both
reading and sending messages.
11. Security considerations
See section "security and trust".
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12. Privacy considerations
See section "security and trust".
13. IANA Considerations
13.1. Creation of the SML registry group
IANA will create a new registry group called "Structured Email
(SML)". This group includes the "SML Email Vocabulary Registry" and
"SML Vocabularies" registries described below.
13.1.1. Creation of the SML Email Vocabulary Registry
IANA will create the following registry:
Registry Name: "SML IETF Email Vocabulary"
Registration Procedure: TBD
13.1.2. Creation of the SML Vocabularies Registry
IANA will create the following registry:
Registry Name: "SML Vocabularies"
Registration Procedure: TBD
(TABLE OF REGISTRATIONS)
The registry entries contain the following fields: - Vocabulary name:
a human readable name for reference - Namespace: the URI namespace
prefix for the vocabulary - Documentation page: a web page with
further documentation about the vocbulary - Scope: the domain
described by that vocabulary - Reference: (IANA) - Notes: any
additional notes
13.1.3. Initial Entries for the SML Vocabularies Registry
The registry initially contains these entries:
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+==========+=====================+=====================+========+=========+=====+
|Vocabulary|Namespace |Documentation page |Scope |Reference|Notes|
|name | | | | | |
+==========+=====================+=====================+========+=========+=====+
|Schema.org|https://schema.org/ |https://schema.org/ |General | | |
| |(https://schema.org/)|(https://schema.org/)|purpose | | |
| | | |concepts| | |
+----------+---------------------+---------------------+--------+---------+-----+
|SML Email |TBD |TBD |Email- |(this | |
|Vocabulary| | |specific|draft) | |
| | | |concepts| | |
| | | |defined | | |
| | | |in IETF | | |
| | | |drafts | | |
+----------+---------------------+---------------------+--------+---------+-----+
Table 1
13.2. Registration of the Content-Purpose MIME header field
The following MIME header field is registered in the Message Headers
registry, as established in [RFC3864]/BCP 90.
*Header Field:* Content-Purpose
*Description:* If the MIME entity is intended as machine-readable
information for the MUA.
*Applicable protocol:* MIME [RFC2045]
*Status:* standards-track
*Author/change controller:* IETF
*Specification document(s):* This document
*Related information:*
13.3. Registration of the $hasStructuredData keyword
The following IMAP/JMAP keyword is registered in the IMAP and JMAP
Keywords registry, as established in [RFC5788].
*IMAP/JMAP keyword name:* $hasStructuredData
*Purpose* Indicate to the client that a message contains structured
data according to this spec.
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*Private or Shared on a server:* SHARED
*Is it an advisory keyword or may it cause an automatic action:* This
keyword can cause an automatic action.
*When/by whom the keyword is set/cleared:* This keyword is set by the
server on delivery.
*Related keywords:* MRM;hasAttachments is peripherally related
*Related IMAP capabilities:* None
*Security considerations:* None
*Published specification:* This document
*Intended usage:* COMMON
*Scope:* BOTH
*Owner/Change controller:* IESG
13.4. Registration of the $MRM keyword
The following IMAP/JMAP keyword is registered in the IMAP and JMAP
Keywords registry, as established in [RFC5788].
*IMAP/JMAP keyword name:* $MRM
*Purpose* Indicate to the client that a message is fully machine-
readable according to this specficiation.
*Private or Shared on a server:* SHARED
*Is it an advisory keyword or may it cause an automatic action:* This
keyword can cause an automatic action.
*When/by whom the keyword is set/cleared:* This keyword is set by the
server on delivery.
*Related keywords:* any message with MRM should have also set
thehasStructuredData keyword
*Related IMAP capabilities:* None
*Security considerations:* None
*Published specification:* This document
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*Intended usage:* COMMON
*Scope:* BOTH
*Owner/Change controller:* IESG
14. Informative References
[HTMLData] WHATWG, "HTML Living Standard: Embedding custom non-
visible data with the data-* attributes",
<https://html.spec.whatwg.org/multipage/dom.html#attr-
data-*>.
[HasAttachment]
IETF imapext WG mailing list, "Registering $hasAttachment
& $hasNoAttachment",
<https://mailarchive.ietf.org/arch/msg/imapext/
MVE5eNHOaNIVGUvN1RKtBL8b278/>.
[JSONLD] W3C JSON-LD Working Group, "JSON-LD 1.1",
<https://www.w3.org/TR/json-ld/>.
[MachineReadable]
NIST, "NIST IR 7511 Rev. 4",
<https://csrc.nist.gov/glossary/term/Machine_Readable>.
[Microdata]
Web Hypertext Application Technology Working Group
(WHATWG), "HTML Living Standard: Microdata",
<https://html.spec.whatwg.org/multipage/#toc-microdata>.
[NC-Itinerary]
Nextcloud GmbH, "Nextcloud Mail: Itinerary support",
<https://github.com/nextcloud/mail/pull/2214>.
[NC-SML] audriga GmbH, "Nextcloud Mail: SML support",
<https://github.com/audriga/nextcloud-mail/tree/enh/sml-
markup-rendering>.
[RC-SML] audriga GmbH, "Structured Email plugin for Roundcube
Webmail",
<https://github.com/audriga/roundcube-structured-email/>.
[RDF] W3C RDF Working Group), "RDF 1.1 Concepts and Abstract
Syntax", <https://www.w3.org/TR/rdf11-concepts/>.
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[RFC2045] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part One: Format of Internet Message
Bodies", RFC 2045, DOI 10.17487/RFC2045, November 1996,
<https://www.rfc-editor.org/info/rfc2045>.
[RFC2046] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part Two: Media Types", RFC 2046,
DOI 10.17487/RFC2046, November 1996,
<https://www.rfc-editor.org/info/rfc2046>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC2183] Troost, R., Dorner, S., and K. Moore, Ed., "Communicating
Presentation Information in Internet Messages: The
Content-Disposition Header Field", RFC 2183,
DOI 10.17487/RFC2183, August 1997,
<https://www.rfc-editor.org/info/rfc2183>.
[RFC2392] Levinson, E., "Content-ID and Message-ID Uniform Resource
Locators", RFC 2392, DOI 10.17487/RFC2392, August 1998,
<https://www.rfc-editor.org/info/rfc2392>.
[RFC3864] Klyne, G., Nottingham, M., and J. Mogul, "Registration
Procedures for Message Header Fields", BCP 90, RFC 3864,
DOI 10.17487/RFC3864, September 2004,
<https://www.rfc-editor.org/info/rfc3864>.
[RFC3987] Duerst, M. and M. Suignard, "Internationalized Resource
Identifiers (IRIs)", RFC 3987, DOI 10.17487/RFC3987,
January 2005, <https://www.rfc-editor.org/info/rfc3987>.
[RFC4021] Klyne, G. and J. Palme, "Registration of Mail and MIME
Header Fields", RFC 4021, DOI 10.17487/RFC4021, March
2005, <https://www.rfc-editor.org/info/rfc4021>.
[RFC5322] Resnick, P., Ed., "Internet Message Format", RFC 5322,
DOI 10.17487/RFC5322, October 2008,
<https://www.rfc-editor.org/info/rfc5322>.
[RFC5598] Crocker, D., "Internet Mail Architecture", RFC 5598,
DOI 10.17487/RFC5598, July 2009,
<https://www.rfc-editor.org/info/rfc5598>.
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[RFC5788] Melnikov, A. and D. Cridland, "IMAP4 Keyword Registry",
RFC 5788, DOI 10.17487/RFC5788, March 2010,
<https://www.rfc-editor.org/info/rfc5788>.
[RFC7515] Jones, M., Bradley, J., and N. Sakimura, "JSON Web
Signature (JWS)", RFC 7515, DOI 10.17487/RFC7515, May
2015, <https://www.rfc-editor.org/info/rfc7515>.
[RFC7942] Sheffer, Y. and A. Farrel, "Improving Awareness of Running
Code: The Implementation Status Section", BCP 205,
RFC 7942, DOI 10.17487/RFC7942, July 2016,
<https://www.rfc-editor.org/info/rfc7942>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[SchemaOrg]
W3C Schema.org Community Group, "Schema.org",
<https://schema.org/>.
[SchemaOrgEmail]
Structured Email, "Schema.org for email",
<https://structured.email/content/related_work/frameworks/
schema_org_for_email.html>.
[WDCStats] Web Data Commons Project, "Web Data Commons - Microdata,
RDFa, JSON-LD, and Microformat Data Sets",
<http://webdatacommons.org/structureddata/#toc3>.
[Yatagarasu]
audriga GmbH, "Yatagarasu Mail",
<https://github.com/audriga/thunderbird-android>.
Author's Address
Hans-Joerg Happel
audriga GmbH
Email: happel@audriga.com
URI: https://www.audriga.com
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