This is easily circumvented by mailer software like this one. It does NOT send mails as coming from the user. It just changes the name:
If I send a mail to this list, I send it from "Marius marius@yo2loj.ro" The mailing list SW will send it from "Marius via 44Net 44net@mailman.ampr.org" Even if that "via 44Net" would not be there, it still is a mailer's address. The same applies if the sender would be "marius@yo2loj.ro 44net@mailman.ampr.org" It would APPEAR it was sent from me, but the actual address is mailman's address.
On the other hand, mail servers are right, one can not send mail to them using their own domain, unless allowed by some trusted source (DNS SFP reccord or similar in the mail host's record) authorizing the external host to send mail in its name. And it is not only yahoo doing this, most mail servers reject such mails. They also reject unknown sender hosts (without RDNS entries), invalid HELLO headers and others. Google is the exception, classifying it as spam instead of just rejecting it with a 554 error.
IMHO this whole thing is actually in place for years, and a non-issue. As long as a mail server is correctly configured, everything works as expected.
Marius, YO2LOJ
On Sat, 10 Nov 2018 at 07:32, Neil Johnson neil.johnson@erudicon.com wrote: (...) Unfortunately this breaks mailing list software like mailman (used for this list) which tries to make e-mail from the list appear as if it is coming from the original sender.
The changes that are required to fix the issue change the functionality of the list software in ways users may not like.