It appears that Jeremy Harris via Exim-users <j...@wizmail.org> said:
On 11/29/23 15:51, John Levine via Exim-users wrote:
Strange but true, sending mail to this list via IPv6 does not work:

2023-11-29 10:35:50.715699500 new msg 271522
2023-11-29 10:35:50.715750500 info msg 271522: bytes 2558 from 
<jo...@taugh.com> qp 83701 uid 82
2023-11-29 10:35:50.726425500 starting delivery 466243: msg 271522 to remote 
exim-users@lists.exim.org
2023-11-29 10:35:56.348847500 delivery 466243: failure:
2a03:4000:0006:b381:0000:0000:0000:0002_does_not_like_recipient./Remote_host_said:_550_unknown_user/Giving_up_on_2a03:4000:0006:b381:0000:0000:0000:0002./STARTTLS_proto=TLSv1.3;_cipher=TLS_AES_256_GCM_SHA384;_subject=/CN=cumin.exim.org;_issuer=/C=US/O=Let's_Encrypt/CN=R3;/
2023-11-29 10:35:56.376048500 end msg 271522

Our log says that message was aimed at exim-us...@cumin.exim.org

Oh, I see the problem. lists.exim.org is a CNAME for cumin.exim.org,
and qmail is standard compliant per RFC 1123:

      5.2.2  Canonicalization: RFC-821 Section 3.1

         The domain names that a Sender-SMTP sends in MAIL and RCPT
         commands MUST have been  "canonicalized," i.e., they must be
         fully-qualified principal names or domain literals, not
         nicknames or domain abbreviations.  A canonicalized name either
         identifies a host directly or is an MX name; it cannot be a
         CNAME.

When I put in an explicit route it uses that rather than resolving the CNAME,
unrelated to IPv6.

It's poor form to use a CNAME as a mail domain, and worse form not to
do what the standard says, but I suppose that horse left the barn a
long time ago.

R's,
John

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