[Bimi] Fwd: bimi Digest, Vol 10, Issue 2

[Marcel Bokhorst <[e-mail]>]

FairEmail will not rely on the BIMI-Location header because this requires
support from the email server. Instead, FairEmail will use the logo in the
VMC and fallback to the DNS record logo location if needed (logo missing in
the VMC, etc). The logo will always be shown, but the verified sender icon
(a double check mark before the sender's name) will be shown only when
everything checks out.

I think it would be a good idea to summarize the steps to follow to
fetch/decode/check BIMI because I think it will make it easier for
developers to implement BIMI and to do this in the right way.

The BIMI TXT record is always being fetched from the top level domain (
example.org). I hope this is correct.

An IMAP flag would be nice, but I guess it will take quite some time until
all email servers are supporting this. Something like a signed
authentication results header would be nice, as long as it can be checked
in a simple and efficient way. For example heavy cryptographic operations
and looking up DNS records (for certificates, etc) would result in slow
downs and increased battery usage when downloading many messages.

Currently, the authentication header inserted by the email server is just
being trusted, but this relies on either inserting the header by the
receiving email server or removing previous headers of intermediate email
servers.

FairEmail will cache the logo for two weeks, which is the same as for
favicons. IMHO this is a reasonable default, preventing logos from being
fetched too often and still allowing to change the logo by the sender
(which I guess won't happen too often).

I hope this clarifies things, else feel free to reply.

Marcel Bokhorst (author of FairEmail)


---------- Forwarded message ----------
From: "Brotman, Alex" <[e-mail]>
To: Trent Adams <tadams=[e-mail]>, "[e-mail]" <
[e-mail]>
Cc:
Bcc:
Date: Thu, 29 Jul 2021 19:45:04 +0000
Subject: Re: [Bimi] Independent BIMI-capable Email Client

I do have that app, but I haven’t yet pointed it at a platform utilizing
BIMI.  I’ll make a note to give it a try.



>From below:



   - They aren't entirely clear on how (or why) to evaluate a logo
   referenced by the BIMI record and the one retrieved from a VMC.



What about the BIMI-Location header instance, if present? (assuming you
want to trust that header, etc).  After that, I’d probably use the VMC if
present.  Should we add an evaluation path to the core document?



   - It's also not all that clear where to retrieve the BIMI logo when the
   email is sent from a subdomain, and match that up with the DMARC results.



I’m not sure I follow the issue here.  Is there some example that shows the
questionable path?



   - There's also the question about whether or not an independent client
   can/should trust the authentication results performed by the mailbox
   provider when deciding to display the BIMI logo.



If you go back to -00 of the core spec, there was an IMAP flag that was
meant to be set (Section 8.6).  That was removed in -01.  I don’t recall
the justification, but may have had to do with MUA workload.  Seems like
the IMAP flag could be reintroduced (POP3 would be an issue though).  Or
perhaps something akin to a DKIM signature signed by the inbound MTA that
could be verified by the MUA.  The latter could potentially be reused
though?



   - There is a non-zero addition of processing required by the client to
   handle BIMI, leading to the question about what impact this may have on a
   client... and what efficiencies can be introduced with local caching (if
   any).



I’d think at least equivalent to the TTL of the BIMI DNS record, perhaps
even up to the expiration of the VMC (though I could understand where a
domain holder could refresh their VMC and change logos, etc).  I’m sure
there’s a better answer somewhere in between.





--

Alex Brotman

Sr. Engineer, Anti-Abuse & Messaging Policy

Comcast



*From:* bimi <[e-mail]> *On Behalf Of *Trent Adams
*Sent:* Friday, July 23, 2021 5:36 PM
*To:* [e-mail]
*Subject:* [Bimi] Independent BIMI-capable Email Client





Since Gmail's announcement, more companies are starting to play around with
BIMI (no surprise there).  And, as expected, the increased attention is
helping to put more eyeballs on the experiment.



And while some of the major mailbox providers are adding BIMI support to
their own mobile clients, I ran into a FOSS project run by a developer in
the Netherlands who has already added BIMI to their independent mobile mail
client.



If you get a chance, I'd suggest taking a look at their FairEmail
implementation as they made some interesting choices:



https://email.faircode.eu/
<https://urldefense.com/v3/__https:/email.faircode.eu/__;!!CQl3mcHX2A!UUyJEN3_Ay0ac-Ux9txqVoPDIfnSSmOT5S-OCXjjRVeXJEEqrPRjGUg2a_oFLJHfdsADeC4$>



The current Android client package can be downloaded from their GitHub
repository:



https://github.com/M66B/FairEmail/releases
<https://urldefense.com/v3/__https:/github.com/M66B/FairEmail/releases__;!!CQl3mcHX2A!UUyJEN3_Ay0ac-Ux9txqVoPDIfnSSmOT5S-OCXjjRVeXJEEqrPRjGUg2a_oFLJHf2gwnAj4$>





So far, they've already found some useful results from their experiment.
And now we're finally at a point where we can test the architecture (and
pressure-test the specifications) beyond the handful of usual suspects.



For example, here're a few issues this work as already surfaced when
interpreting the specifications [1] [2] [3]:



   - They aren't entirely clear on how (or why) to evaluate a logo
   referenced by the BIMI record and the one retrieved from a VMC.
   - It's also not all that clear where to retrieve the BIMI logo when the
   email is sent from a subdomain, and match that up with the DMARC results.
   - There's also the question about whether or not an independent client
   can/should trust the authentication results performed by the mailbox
   provider when deciding to display the BIMI logo.
   - There is a non-zero addition of processing required by the client to
   handle BIMI, leading to the question about what impact this may have on a
   client... and what efficiencies can be introduced with local caching (if
   any).



What I found most interesting was how the specifications assume a tight
coupling between the client and the mailbox provider.  It definitely opens
the question of whether it's possible (or recommended) for an independent
client to support BIMI even for email received by mailbox providers that
don't support it.  It's real-world interop testing like this that I hope
will highlight issues that need to be discussed as the experiment continues.



Also, if anyone would be willing to test the FairEmail client, I'd be keen
to hear if you think that the way it leverages BIMI matches your
expectations.  Also, are there any others out there that might contribute
their learnings to the conversation?



Anyway, thanks for any feedback you may have.  It'll be useful to hash out
ideas for improvements and next steps on the list.



Cheers,

Trent



[1] https://datatracker.ietf.org/doc/html/draft-blank-ietf-bimi-02
<https://urldefense.com/v3/__https:/datatracker.ietf.org/doc/html/draft-blank-ietf-bimi-02__;!!CQl3mcHX2A!UUyJEN3_Ay0ac-Ux9txqVoPDIfnSSmOT5S-OCXjjRVeXJEEqrPRjGUg2a_oFLJHfVUyABq4$>

[2]
https://datatracker.ietf.org/doc/html/draft-fetch-validation-vmc-wchuang-00
<https://urldefense.com/v3/__https:/datatracker.ietf.org/doc/html/draft-fetch-validation-vmc-wchuang-00__;!!CQl3mcHX2A!UUyJEN3_Ay0ac-Ux9txqVoPDIfnSSmOT5S-OCXjjRVeXJEEqrPRjGUg2a_oFLJHfIMZn7Z8$>

[3]
https://datatracker.ietf.org/doc/html/draft-brotman-ietf-bimi-guidance-03
<https://urldefense.com/v3/__https:/datatracker.ietf.org/doc/html/draft-brotman-ietf-bimi-guidance-03__;!!CQl3mcHX2A!UUyJEN3_Ay0ac-Ux9txqVoPDIfnSSmOT5S-OCXjjRVeXJEEqrPRjGUg2a_oFLJHfQyodXbk$>





-- 

*J. Trent Adams*

*Director, Ecosystem Security*

*Proofpoint*



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