I set up a gmail forward to yahoo and the forward was blocked by yahoo servers with a 554 5.7.9 error "permanent error." This is yahoo's definition of the 554 error:
A 553 or 554 SMTP error indicates an email could not be delivered due to a permanent problem.
Message delivery can be permanently deferred because:
* You're trying to send a message to an invalid email address.
* Your message failed authentication checks against your sending domain's DMARC or DKIM policy.
* The message contains characteristics that Yahoo won't accept for policy reasons.
* Other suspicious behavior which leads Yahoo to issue a permanent rejection for your SMTP connection.
* Your IP is listed by Spamhaus. Please check with https://www.spamhaus.org
If you consistently receive 5xx errors when sending to Yahoo, we encourage you to review our Sender Best Practices, since 5xx errors can be a symptom of a more widespread, general problem.
You should not retry sending an email that comes back with with a 5xx error. List managers should have a policy for removing email addresses that generate 5xx errors/bounces.
Hope this helps anyone.