Strange error 554 with mail sent to yahoo

[smallwheels]

I got an error from yahoo in the mail error log that made me wonder:

The mail system

<$a_user_mailaddress@yahoo.com>: host mta7.am0.yahoodns.net[98.136.96.77] said: 554
Message permanently deferred due to unresolvable RFC.5321 from domain. See
https://senders.yahooinc.com/error-codes#unresolvable-from-domain (in reply
to end of DATA command)


Final-Recipient: rfc822; $a_user_mailaddress@yahoo.com
Original-Recipient: rfc822; $a_user_mailaddress@yahoo.com
Action: failed
Status: 5.0.0
Remote-MTA: dns; mta7.am0.yahoodns.net
Diagnostic-Code: smtp; 554 Message permanently deferred due to unresolvable
RFC.5321 from domain. See
https://senders.yahooinc.com/error-codes#unresolvable-from-domain

The provided link explains:

Unresolvable RFC.5321 or RFC.5322 from domain

• These errors indicate that the domain used to the right of the @ in the MAIL FROM or HEADER FROM does not appear to be valid.
• We determine if the domain name exists by using an SOA query; therefore, if multiple subdomains are used in MAIL FROM or HEADER FROM commands, then besides setting up a DNS A or MX record (perhaps using a wildcard), then SOA records must be set up as well.
• These errors can be returned as either a timeout/servfail (451) or a permfail (554)

This does not seem to make any sense, especially as this mail went to a freshly registered user as mail-notification for the Welcome Message he received via PN in the forum immediately after registration. For this to be possible the user must have received the email with the account confirmation link before that and clicked on it successfully. So there was sent another email to this very same user immediately before this one, there is no error message in the log for that first mail, so I assume it must have gone through.

I am on 2.3.10, use the mailer built into XF (sending mails directly from the server w/o relay) and do have bounce-handling set up with a dedicated mail address.

Obviously my from-domain is resolvable (as it is my forum domain and there are mails sent via the mail system sent constantly and successfully, including to yahoo addresses) and other than automated forum mails mails are sent and received via this domain. The bounce mail address does live on a own, dedicated subdomain, but this should not be an issue (and hasn't been one until now).

Does anyone have an explanation what has happened and why and how to work around this?

PS: As a sidenote the XF bounce-handler treated this as a soft bounce where it seems to be a hard bounce. However, the wording of yahoo's message is ambiguous and leaves a lot to be desired. What on earth is Message permanently deferred supposed to mean? Either it is deferred (then it will be dealt with it later) or it is a permanent error. The combination "permanently deferred" makes no sense at all to me and basically sounds like a very lazy mail system. ;-)