Same issue here.
I even filled out the form that yahoo suggests we fill in if we want to send email to their servers:
Aside from a standard response saying that they can't guarantee delivery I didn't get anything from them.
My error is this:
150D6125B8B 90331 Fri Jan 16 09:53:14 firstname.lastname@example.org
(host mta5.am0.yahoodns.net[220.127.116.11] said: 421 4.7.1 [TS03] All messages from 18.104.22.168 will be permanently deferred; Retrying will NOT succeed. See http://postmaster.yahoo.com/421-ts03.html (in reply to MAIL FROM command))
Same problem with Yahoo, mails just not getting through the recipient get an email from Yahoo saying Message not accepted for policy reasons, which ironically lands in the junk box! Here's the log:
relay=mx-eu.mail.am0.yahoodns.net[22.214.171.124]:25, delay=561, delays=561/0.02/0.04/0.01, dsn=4.7.0, status=deferred (host mx-eu.mail.am0.yahoodns.net[126.96.36.199] said: 421 4.7.0 [TSS04] Messages from 188.8.131.52 temporarily deferred due to user complaints - 184.108.40.206; see https://help.yahoo.com/kb/postmaster/SLN3434.html (in reply to MAIL FROM command))