Hello Kirill.

Kirill A. Korinsky wrote in
 <98c1f03bc1d6c722@mx1.catap.net>:
 ...
 |> Kirill A. Korinsky wrote in
 |>  <2fdd33f2325e6817@mx2.catap.net>:
 |>|>|I imply that using ed25519 usually leads to malformed signature, \
 ...
 |>|> Then these are not standard compliant.  The DKIM standard 6376
 |>|> *explicitly* supports multiple signatures.
 |>|
 |>|Yes, RFC may imply that but OpenDKMI was released quite a while ago \
 |>|and the
 |>|last stable release seems that doesn't handle well this case.
 |> 
 |> OpenDKIM cannot.  I looked at its code in about January and there
 |> is no notion of it.  zdkimfilter as of courier bases upon it, and
 |> supports it.  (Very preprocessor sprinkled crypto code in between
 |> several libraries that uses, though, and the OpenSSL 3.0 thing
 |> even fiddles with openssl parameters which i have *not*
 |> understood from my short glance..)
 |
 |And here the issue and my point: until OpenDKIM is supporting anything else
 |than RSA may lead to delivery emails into Junk.

No, Kirill, you are misunderstanding a little bit how DKIM works.

  ...
 |> I could very much imagine that if you change to RSA-SHA256 then
 |> your problem will vanish.
 |
 |Nope, it doesn't
 |
 |See mxtoolbox [1] for the case of RSA-SHA256 and icloud says, let me quote:
 |
 |  Authentication-Results: dkim-verifier.icloud.com; dkim=permerror \
 |  (0-bit key) header.d=korins.ky header.i=@korins.ky header.b=VNwI9oir
 |  Authentication-Results: dkim-verifier.icloud.com; dkim=pass (2048-bit \
 |  key) header.d=korins.ky header.i=@korins.ky header.b=qwDQ6QCD
 |
 |The issue that one of signatures is invalid, and icloud moves mail to the
 |Junk folder.

Because DKIM says that as long as *one* signature passes
correctly, DKIM has succeeded.  The introduction of new algorithms
and key changes etc is quite broadly foreseen in a lot of RFCs
regarding public key infrastructure in the last two decades (my
view is very limited however, but, still..).

 |As soon as I use only RSA signatures, emails are delivered to inbox.

This is broken behaviour of these people, see RFC 6376, 6.1:

      INFORMATIVE NOTE: The rationale of this requirement is to permit
      messages that have invalid signatures but also a valid signature
      to work.
      ...
      the message should succeed even in the presence of the
      known-broken signature.

What they are doing is wrong.  Maybe if you move it out of Junk
a few times their algorithm learns or what, i do not know.
I would start screaming, but normally noone listens anyhow, sure.

 |Footnotes:
 |[1]  https://mxtoolbox.com/deliverability/86e2b0ff-ba95-47f3-b71e-4ead73\
 |653a73

Ah, you, i do not look, this required Javascript and whatnot.

--steffen
|
|Der Kragenbaer,                The moon bear,
|der holt sich munter           he cheerfully and one by one
|einen nach dem anderen runter  wa.ks himself off
|(By Robert Gernhardt)