Network Working Group A. Vesely Internet-Draft November 2020 Intended status: Informational Expires: 20 May 2021 Mailing List Manager (MLM) Transformations draft-vesely-dmarc-mlm-transform-00 Abstract The widespread adoption of Domain-based Message Authentication, Reporting, and Conformance (DMARC) led Mailing List Managers (MLM) to rewrite the From: header field as a workaround. This document describes cases where it is possible to revert MLM transformations and hence verify DomainKeys Identified Mail (DKIM) signatures that were applied at submission time. For reliable results, some compliance is required of all agents involved, author domain signers, MLMs, forwarders, and final recipients. MLM transformation reversion reduces the DMARC's effects on indirect mail flows. 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 5 May 2021. Copyright Notice Copyright (c) 2020 IETF Trust and the persons identified as the document authors. All rights reserved. Vesely Expires 20 May 2021 [Page 1] Internet-Draft MLM Transformations November 2020 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. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 2. Terms Definitions . . . . . . . . . . . . . . . . . . . . . . 3 3. Revertible Transformations . . . . . . . . . . . . . . . . . 3 3.1. Header Transformations . . . . . . . . . . . . . . . . . 3 3.2. Body Transformations . . . . . . . . . . . . . . . . . . 4 4. Outline of a Reverting Verifier . . . . . . . . . . . . . . . 4 5. Actors Roles and Compliance . . . . . . . . . . . . . . . . . 6 5.1. Original Signer . . . . . . . . . . . . . . . . . . . . . 6 5.2. MLM . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 5.3. Verifier . . . . . . . . . . . . . . . . . . . . . . . . 8 6. Security Considerations . . . . . . . . . . . . . . . . . . . 8 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9 7.1. Permanent Message Header Field Names . . . . . . . . . . 9 8. References . . . . . . . . . . . . . . . . . . . . . . . . . 9 8.1. Normative References . . . . . . . . . . . . . . . . . . 9 8.2. Informative References . . . . . . . . . . . . . . . . . 10 Appendix A. Examples . . . . . . . . . . . . . . . . . . . . . . 11 A.1. Single-part plain text . . . . . . . . . . . . . . . . . 11 A.2. Multipart added . . . . . . . . . . . . . . . . . . . . . 12 A.3. Multipart wrapped . . . . . . . . . . . . . . . . . . . . 14 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 15 1. Introduction Mailing List Managers (MLMs) can be configured to add a footer and a subject tag to the messages that they redistribute. Although that behavior slightly exceeds the very limited set of modifications and actions described by Section 3.9.2 of [RFC5321], it is a welcome, time-honored tradition. According to their configuration, the modifications they carry out on messages may result in a set of stylized transformations that are programmatically revertible. Reversion allows to verify DomainKeys Identified Mail (DKIM) signatures ([RFC6376]) that were applied before the transformation. Domain-based Message Authentication, Reporting, and Conformance (DMARC) ([I-D.ietf-dmarc-dmarcbis]) hinges on the alignment of the domain in the From: header field with a verified DKIM signature. For that reason, MLMs that transform messages have to rewrite From:. A deed which can be mitigated in some cases. Vesely Expires 20 May 2021 [Page 2] Internet-Draft MLM Transformations November 2020 Mailbox providers can configure their mail submission agents (MSAs) in order to ease MLM transformation reversion. Or they can make it impossible. It is their policy and their will. MLM operators make a similar decision. When they both agree on revertibility, a well equipped receiver can verify the original signatures. The outcome is twofold: 1. Author domains receive positive feedback about DKIM verification of mailing list traffic. That might eventually lead them to harden their DMARC policy. 2. Final recipient's mail delivery agents (MDAs), which know by the Authentication-Results: field whether a rewritten From: header was verified, can safely undo From: munging (after any external forwarding). 2. Terms Definitions *Signers* and *verifiers* are defined in [RFC6376]. The use of the term *Mailing List Manager*, almost always abbreviated *MLM* follows [RFC6377]. A MLM is a kind of *Mediator* in [RFC5598] parlance. *Message* is defined in [RFC5322]. It consists of a *header* made up of one or more *fields* and a *body*, possibly composed of various MIME *entities*, the latter being defined in [RFC2045] and companions. The term *original* is used here to refer to the Author or parts of the Author's message as it was sent out by the Author's domain, where *Author* is defined in [RFC5598]. 3. Revertible Transformations Message modifications can affect the header and/or the body of a message. This document only considers the very limited set of transformation described in the following subsections. They turn out to be revertible. 3.1. Header Transformations MLM often modify the Subject: field by inserting a tag at the beginning of its value. A tag consists of a short text delimited by square brackets. For example:: Subject: [added tag] Original value of subject This transformation is easily reverted by removing the tag. For security reasons, subject tags must not exceed 20 characters. Vesely Expires 20 May 2021 [Page 3] Internet-Draft MLM Transformations November 2020 A more recent modification carried out by MLMs is From: rewriting. It alters the value of From: in order to pass DMARC filters. MLMs save the original value of From: in a variety of places, including Reply-To:, Cc:, X-Original-From:. When the original value is known, the transformation is revertible. 3.2. Body Transformations Footer addition is often performed in one of three ways, according to the format of the original message. Single-part plain text When the original message is not structured, a footer can be appended at the end of the original text. See example in Appendix A.1 Multipart added The footer stands in its own MIME entity, which is appended as the last part of an original multipart/mixed structure. See example in Appendix A.2 Multipart wrapped The footer stands in the second entity of a new multipart/mixed MIME structure whose first entity consists of the original body. See example in Appendix A.3 The footer begins with a line consisting exclusively of underscore ("_", ASCII 95) characters, at least four of them. For security reasons, the footer must belong to an entity of Content-Type: text/ plain in all cases. In addition, footers cannot exceed 10 lines of text, each shorter than 80 characters. 4. Outline of a Reverting Verifier The algorithm described here is implemented in a mail filter. It usually reads the input message twice -first pass, verify; last pass, write Authentication-Results and the rest of the message to follow. When enabling MLM transformation reversion, there can be a retry pass in between those two. The result is yielded during the SMTP dialogue with no noticeable delay. Implementing reversion changed the software from 22730 lines of C code to 26762. The bulk of such ~18% increase is due to the addition of encoding conversion functions. Changes involve both verifying and signing functions (see Section 5.1 for the latter). While reading the header in the first pass, the verifier looks for specific fields: Vesely Expires 20 May 2021 [Page 4] Internet-Draft MLM Transformations November 2020 * From: * Original-From: * X-Original-From: * Reply-To: * Cc: These are candidates to the original mailbox. The verifier also collects the Subject: and any field named Original-* that the original signer might have set to ease the reversion. At the end of the header, candidate original mailboxes are sorted according to the display name, which MLMs try and keep unaltered. The best candidate is then added to the collected set of Original-* fields. If the Subject: begins with a tag, its version without tag is added to that set as well, unless one is there already. Next, before reading the body, the verifier looks for prospect signatures; that is, signatures whose "d=" domain is not aligned with SPF credentials ([RFC7208]), List-Post: ([RFC4201]), Sender:, or the rewritten From: (if deemed to have been rewritten). If any such signature exist, along with MLM or other signatures, then the verifier enables parsing the body to look for a footer. Body parsing is done in parallel with body canonicalization during the first pass. For multipart, track top level entities. Set transformation type to "wrapped" if there are exactly two entities, "added" otherwise. For single-part, body parsing must avail of encoding conversions as needed. Assume identity encoding, 7bit or 8bit, unless otherwise directed by an Original-Content-Transfer- Encoding: field. At the end of the first pass, the verifier knows how prospect signatures did. Let's recall that DKIM signature verification results from two independent operations, steps 3 and 4 in Section 6.1.3 of [RFC6376]. The signature in the "b=" tag depends on the header, while the body hash in the "bh=" tag depends on the body: * If the signature "b=" did not verify and the set of Original-* fields is not empty, then it is worth to try and re-canonicalize the header using the values in the set of Original-* fields. Vesely Expires 20 May 2021 [Page 5] Internet-Draft MLM Transformations November 2020 * If the body hash "bh=" did not match and a footer was found, then it is worth to try and re-canonicalize the body excluding the footer. None, one, or both of the above operations are performed in the retry pass. On writing Authentication-Results, if a prospect signature verifies after replacing the From: field, the verifier writes a prominent, well documented "reason" in the relevant resinfo stanza (Section 2.2 of [RFC7601]). That way, reversion elements can be easily recognized and parsed by downstream agents. 5. Actors Roles and Compliance 5.1. Original Signer Signers who wish their users to be able to participate to mailing lists can adopt rules apt to ease MLM transformations reversion. Doing so can slightly weaken DKIM'S stiffness, and expose to the risk of malicious MLMs. A sender that doesn't know which of its mail recipients are likely to be MLMs might abide by the following rules for all outgoing mail, in the expectation that few of its users correspondents are likely to be malicious. A sender that had some idea which recipients are MLMs could apply the rules only to mail to those recipients. Or a sender might apply the rules to all mail except that sent to recipients with poor reputations. A special rule is the addition of an Original-From: header field with a value identical to the one signed in From:. Original-From: is defined by [RFC5703] in the context of Sieve Email Filtering. As Sieve operates at time of final delivery, DKIM verifiers which act at the time of message transit can reliably use it. Original-From: is special because verifiers may infer that the field was added by the original signer rather than by MLMs. In that case, they can send DMARC feedback reports to the original signer even if From: was rewritten. Other generic rules to ease reversion are as follows: * DKIM signatures must deploy the "relaxed" canonicalization, at least for the header, since MLMs may reflow header fields. * The quoted-printable encoding must not be used for the body of single-part text/plain messages, as it is impossible to guess original soft line breaks after re-encoding. Base64 is much more robust. Vesely Expires 20 May 2021 [Page 6] Internet-Draft MLM Transformations November 2020 * Single-part text/plain messages encoded as base64 must follow a constant column width of 76 characters. The encoding must be advertised by adding a new header field as follows: Original-Content-Transfer-Encoding: base64 * If the original Subject: begins with a tag, its value must be copied to an Original-Subject: header field. The latter field is also defined by [RFC5703], and the same usage considerations hold. * Content-Type: and Content-Transfer-Encoding: are fields related to the data form. Mailers often rewrite them, so they should not be signed. If signed, their Original- counterpart should be set too. * When signing Cc: or Reply-To:, add their Original- counterparts to the header, as MLMs are likely to change them. * Original-*: fields with an empty value stand for non-existing counterparts. * Original-* fields need not be signed. If original signatures can be recovered, that suffices; otherwise, the unverified signature is irrelevant. 5.2. MLM Participating MLMs must not operate transformations other than those listed in Section 3. Since DKIM is MIME-agnostic, attention must be paid to preserve the exact preamble and epilogue of the original MIME structure. MLMs must apply their own DKIM signature. The presence of signatures by multiple domains can be used by verifiers to infer that a message underwent MLM transformations. MLMs must not set the Original-From: field, which is reserved to original signers. It is recommended that MLMs add a mailbox entry to Reply-To: or Cc: in order to ease off-list replies as well as to allow transformation reversion, but only in case the Original-From: is missing. MLMs may set Original-* fields other than Original-From:, but only if the original message contains no Original-* field at all. That is, when the author's domain is not aware of the possibility to ease MLM transformation reversion. Vesely Expires 20 May 2021 [Page 7] Internet-Draft MLM Transformations November 2020 MLMs which collect posts from other MLMs must avoid to add their own footer and subject tag. Transformation reversion cannot be stacked. A second-level MLM can modify or replace the content of previous transformations. Attention must be paid to not exceed tag and footer length limits. 5.3. Verifier Attempts to verify original signatures can be done as outlined in Section 4. The reversion must not replace the messages signed and distributed by MLMs, with one exception detailed in the next paragraph. Only the result of the verification is written out. If an original signature with rewritten From: is recovered, the verifier must make sure that an Original-From: field with the verified mailbox is written out. An MDA downstream may combine the Authentication-Results: and Original-From: fields to restore the original value of From:. This is the only recommended modification to the distributed message. It must be done after any dot-forward processing, so that external verifiers receive the message as distributed by the MLM, and can revert transformations by themselves. If the Original-From: is set, the corresponding DMARC record may be looked up and its "rua=" and "ruf=" tags considered for feedback reports. If DMARC policies are considered, it is the the From: field which rules, not the Original-From: nor any other mailbox value, unless verified. 6. Security Considerations Rewriting the From: header field is an unwelcome modification to messages. It fosters the belief that the display name of a mailbox is more trustworthy than the angle address. A belief further consented by the tendency to not even display the latter. Bad actors take advantage of this belief by displaying the names of trusted institution paired with trash email addresses hidden between angle brackets. That trick defeats DMARC's purpose. It is out of this document's scope to suggest how mail user agents (MUAs) could counter phishing by highlighting security indicators (for the extent that indicators can actually help preventing phishing attacks). Let's just note that MUAs have to cope with MLM and phishing alike, which makes it hard to devise a pattern to tell apart one from the other without getting involved with the reputation of the specific domains. Vesely Expires 20 May 2021 [Page 8] Internet-Draft MLM Transformations November 2020 By safely restoring munged From: to the original value, that contrast is eliminated. Then, perhaps, deceptive mailboxes might become amenable to some kind of efficient indication. Of course, MLM role can be played by miscreants as well. However, replaying a signed message, even with revertible transformations, has more limits than forging scam messages anew. Therefore, the risk introduced by easing transformation reversion is considerably lower than that of not signing, or of keeping DMARC policy at "none". Compared with the use of "l=" tag (Section 8.2 of [RFC6376]), the fact that footers are written in plain text removes the main security objection about footer additions. Namely, footers cannot completely replace the original content in the end recipient's eyes by exploiting lax HTML parsing in the MUA. Still, a footer can contain dangerous URLs and deceiving text. That possibility has to be countered by usual mail filtering and savvy behavior. 7. IANA Considerations IANA maintains the "Message Header" registry with several subregistries. IANA is asked to make the assignments set out in the following section. 7.1. Permanent Message Header Field Names IANA is asked to create new entries in the "Permanent Message Header Field Names" registry as follows. +===================+==========+==========+==========+===========+ | Header Field Name | Template | Protocol | Status | Reference | +===================+==========+==========+==========+===========+ | Original-Content- | | mail | standard | this I-D | | Transfer-Encoding | | | | | +-------------------+----------+----------+----------+-----------+ | Original-Reply-To | | mail | standard | this I-D | +-------------------+----------+----------+----------+-----------+ | Original-Cc | | mail | standard | this I-D | +-------------------+----------+----------+----------+-----------+ Table 1 8. References 8.1. Normative References Vesely Expires 20 May 2021 [Page 9] Internet-Draft MLM Transformations November 2020 [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, . [RFC5321] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321, DOI 10.17487/RFC5321, October 2008, . [RFC5322] Resnick, P., Ed., "Internet Message Format", RFC 5322, DOI 10.17487/RFC5322, October 2008, . [RFC6376] Crocker, D., Ed., Hansen, T., Ed., and M. Kucherawy, Ed., "DomainKeys Identified Mail (DKIM) Signatures", STD 76, RFC 6376, DOI 10.17487/RFC6376, September 2011, . [I-D.ietf-dmarc-dmarcbis] Gustafsson, E., Herr, T., and J. Levine, "Domain-based Message Authentication, Reporting, and Conformance (DMARC)", Work in Progress, Internet-Draft, draft-ietf- dmarc-dmarcbis-00, 11 November 2020, . 8.2. Informative References [RFC4201] Kompella, K., Rekhter, Y., and L. Berger, "Link Bundling in MPLS Traffic Engineering (TE)", RFC 4201, DOI 10.17487/RFC4201, October 2005, . [RFC5703] Hansen, T. and C. Daboo, "Sieve Email Filtering: MIME Part Tests, Iteration, Extraction, Replacement, and Enclosure", RFC 5703, DOI 10.17487/RFC5703, October 2009, . [RFC5598] Crocker, D., "Internet Mail Architecture", RFC 5598, DOI 10.17487/RFC5598, July 2009, . [RFC6377] Kucherawy, M., "DomainKeys Identified Mail (DKIM) and Mailing Lists", BCP 167, RFC 6377, DOI 10.17487/RFC6377, September 2011, . Vesely Expires 20 May 2021 [Page 10] Internet-Draft MLM Transformations November 2020 [RFC7208] Kitterman, S., "Sender Policy Framework (SPF) for Authorizing Use of Domains in Email, Version 1", RFC 7208, DOI 10.17487/RFC7208, April 2014, . [RFC7601] Kucherawy, M., "Message Header Field for Indicating Message Authentication Status", RFC 7601, DOI 10.17487/RFC7601, August 2015, . Appendix A. Examples In the examples that follow, the first character of each wrapped line of DKIM-Signature: fields should be a TAB. For editorial reasons, it is rendered as four spaces. While visually there is little difference, those signatures won't verify unless replacing them with a TAB. To verify the examples, public keys can be set as follows: s._domainkey.example.com IN TXT ( "v=DKIM1; g=*; k=rsa; " "p=MIGfMA0GCSqGSIb3DQEBAQUAA4GNADCBiQKBgQCqlye7m5zLLXoIpBp2OO05LNMqK" "u0zKowoHOpyRpviOVqOaNCk5uZ+wY00JwrKbt5u1G1ghuXsFkFkl0h00LBurz7ivyZH" "3LohSWOZ8okgR+8kuGu9GHtQ+MqgRd16tlCF8PlWS2kGaBQKua1zk+ZCDwFy82Uo5G2" "1nu/+Nn2sUwIDAQAB" ) s._domainkey.lists.example IN TXT ( "v=DKIM1; k=rsa; " "p=MIGfMA0GCSqGSIb3DQEBAQUAA4GNADCBiQKBgQDgnLb2TZ6KECBMBo9ZLqDFt4ZBz" "NHFrgBj/LVJVFU8IQP8uH4G8Pj0mEHRo1qpf0vuFI2HVpe/3NhzkT4Ay/1ZIIsxY754" "f2thlhBvKh4AAgZFmzRvA3aZs6Tb/ERmD+a51liEMFaTOmY4mWeLi9wOM51usQ9Q65i" "8IP/vjHM3rQIDAQAB" ) A.1. Single-part plain text Base64 encoding has to be decoded in order to locate the footer. The original encoding was text/plain, this can be inferred by the verifier from the absence of an Original-Content-Transfer-Encoding: field. The original body hash will match after decoding and removing the footer. Note that an "l=" tag couldn't have done the trick in this case. Vesely Expires 20 May 2021 [Page 11] Internet-Draft MLM Transformations November 2020 Received: from lists.example by subscriber.example.org with ESMTP DKIM-Signature: v=1; a=rsa-sha256; c=simple/simple; d=lists.example; s=s; t=1603901305; bh=MjC5ikx26j8beyDJiz7Rk/4W+ppdGOmqh6koz0gLa8o=; h=Date:From:To:Subject; b=PNIYHGd7aytHEvew44WRpSfl4Py3c/9mKjovvQ1ps/xdpkl1/z+gWeu8e8ZmR7gdE iT2TsJ7ni3Lfp5oUpGCko5MvCoqcKX7Zmq3CmXTxRTwwvVZrAp/ir8UTvG+rJFnyEZ Yi3dSTX4rKe2LotyLkqcs+/uXaWEADbqcBp/9iHo= Received: from mail.example.com by lists.example with ESMTP DKIM-Signature: v=1; a=rsa-sha256; c=simple/simple; d=example.com; s=s; t=1603889142; bh=hrDXocZNPy1+eUFYIk1PVRKa6mUMb8+ql9CFNABacww=; h=Date:From:To:Subject; b=YFLwvvW5bGbE5HpJwBM1JoL1F9b8AxdVFlwE/vOkL0p/pPpr7g9KnPXqwoEXZgFI0 /kkTHK/Afy4gaWZQfwDZ77LuxYSMFjwpNorSc0YEGzHYzLCN7rL1e+xE7B7kOCThiq ebaMdcaHeZF6QUmWcUkEj8LVkxrvWi+bTzd3RnaA= Original-From: Author Received: from mua.example.com by mail.example.com with ESMTPA Message-ID: <123456@author.example> Date: Mon, 28 Oct 2020 13:12:55 +0100 From: Author MIME-Version: 1.0 To: MLM@lists.example Subject: [example] Check simple MLM message Content-Type: text/plain; charset=us-ascii Content-Transfer-Encoding: base64 VGhpcyBpcyBhIHBsYWluIHRleHQgbWVzc2FnZSBzdWJtaXR0ZWQgdG8gYSBtYWlsaW5nIGxpc3Qu ClRoZSBtYWlsaW5nIGxpc3QgaXMgZXhwZWN0ZWQgdG8gYWRkIGEgZm9vdGVyIGFuZCBhIHN1Ympl Y3QgdGFnLgoKQmVzdApBdXRob3IKCl9fX19fX19fX19fX19fX19fX19fX19fX19fX19fX19fX19f X19fX18KdGhpcyBtZXNzYWdlIHdhcyBtb2RpZmllZCBieSBNTE0gZXhhbXBsZQphZGRpbmcgdGhp cyBmb290ZXIgYW5kIHRoZSBzdWJqZWN0IHRhZwoobm90ZSB0aGF0IGw9IGlzIG5vdCBzZXQpCg== A.2. Multipart added When the original message has a MIME structure, MLMs can append an entity. Received: from lists.example by subscriber.example.org with ESMTP DKIM-Signature: v=1; a=rsa-sha256; c=simple/simple; d=lists.example; s=s; t=1603974193; bh=sEPYSlJlh90leqy5+63oPn1iU+9P684R92cZHXa9ENw=; h=Date:From:To:Subject; b=fTSAMcaEatofQCuAeUhlTXmVl5j9bPbwWgc84NWtoSt5zT+SSNp37DTzhYIGHozEk bpldArGQ+GygJE1b2witi6NctBd1O/xsUwDcJQxDXkF63QlCcalbKWypHZOhRqncUQ zgUzdcuYgqTYMJ0NoTP8fqu0HdgmjD2LJXjV3pVI= Old-Authentication-Results: lists.example; dkim=pass header.d=example.com Received: from mail.example.com by lists.example with ESMTP DKIM-Signature: v=1; a=rsa-sha256; c=simple/simple; d=example.com; s=s; t=1603973996; bh=eWqyE53pjRVCFGyHY1zGQTkCEvucN1vNN4cTcWk90WU=; Vesely Expires 20 May 2021 [Page 12] Internet-Draft MLM Transformations November 2020 h=Date:From:To:Subject; b=LGP1M3IX6XORfLs8HRLCFOcymzsPn+8+ljgQlmeNlCC/2Cl1+aBDCIEnzWI0pceCb zg32vFfEeryvRDHB1L1K4rrKCEznvO0J3p1xkUPEWpSpzxUGw+PK9KA9ePZ5qdz7cI /hXf7zjebznNdDQJnxajf7QHnx1tXmxijsJ1jiGQ= Old-Authentication-Results: example.com; auth=pass (details omitted) Original-From: Author Received: from mua.example.com by mail.example.com with ESMTPA Message-ID: <123456@author.example> Date: Mon, 28 Oct 2020 13:12:55 +0100 From: Author via MLM MIME-Version: 1.0 To: MLM@lists.example Subject: [example] Check simple MLM message Content-Type: multipart/mixed; boundary=original-boundary Original preamble must be preserved! --original-boundary Content-Type: text/plain; charset=us-ascii Content-Transfer-Encoding: 7bit This is a plain text message submitted to a mailing list. The mailing list is expected to add a footer and a subject tag. Best Author --original-boundary Content-Type: image/png Content-Transfer-Encoding: base64 iVBORw0KGgoAAAANSUhEUgAAAAYAAAAGCAYAAADgzO9IAAAABHNCSVQICAgIfAhkiAAAAAlwSFlz AAAHKgAAByoB49HU1wAAABl0RVh0U29mdHdhcmUAd3d3Lmlua3NjYXBlLm9yZ5vuPBoAAAB+SURB VAiZNcGxDYUgAEXRhxTMYWLFVlDTOAUjOIEzWDqEC1igCQ0LSLi/+ueotUZKieu6uO+bdV2ptaLz PDHGsG0b+74jieM40Pd91Fr5K6UAMC3LImutxhgaY8g5p3meNcUYFULQ+756nkchBMUYpd47OWe8 93jvyTnTe+cHXqRZbKSV4EoAAAAASUVORK5CYII= --original-boundary Content-Tyep: text/plain ________________________________________ this message was modified by MLM example adding this footer and the subject tag (note that l= cannot work in this case) --original-boundary-- Vesely Expires 20 May 2021 [Page 13] Internet-Draft MLM Transformations November 2020 A.3. Multipart wrapped When the original body is multipart/alternative, MLMs have to wrap the whole body into the first entity of a multipart/mixed structure. Indeed, appending an entity to a multipart/alternative would result in it either hiding or being hidden by the existing ones. 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The mailing list is expected to add a footer and a subject tag. Best Vesely Expires 20 May 2021 [Page 14] Internet-Draft MLM Transformations November 2020 Author --original-boundary Content-Type: text/html;

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--original-boundary-- Original epilogue --MLM-boundary Content-Type: text/plain ________________________________________ this message was modified by MLM example adding this footer and the subject tag (note that l= is not set) --MLM-boundary-- MLM epilogue Author's Address Alessandro Vesely v. L. Anelli 13 20122 Milano MI Italy Email: vesely@tana.it Vesely Expires 20 May 2021 [Page 15]