1. The apple issue you have raised
-- The Apple mail.app sends an invalid HELO, as evidenced by this in this log:
dispatching EHLO smtpclient.apple
Apple said it not a problem for them and there is nothing to fix.
2. I agree that yahoo is the source of the issue, not SME 10. ReetP wrote "Just wondering if Yahoo got blocked due to a blocked spamhaus query by your SME?" That is possible but I am not sure where to look for that.
3. First, check the incoming mail from them
-- I looked in the logs on SME. There are a good number of inbound email from yahoo passing muster on SME, but no sign of the email from my buddy.
4. are you using alternative DNS for your server eg Google or OpenDNS?
-- No. SME is set to its default for DNS.
5. There is a thread here on using PiHole
-- I will look into this
6. your line says that the mail to your buddy was denied because it does not allow relay. SME does not allow relay if you are not authenticated.
-- Understood. This seems odd because the 550 message was issued by yahoo and the notice sent to my buddy.
---- The flow of notes: I sent a note to my buddy. He replied to it. Yahoo complained, did not send the reply to me and returned a notice to my buddy.
As ReetP pointed out, there is more than one issue in this one post. I apologize for that.
1. yahoo's "open resolver" complaint
2. The check_goodrcptto: recipient mybuddy@gmail.com denied I found in the mail log.
Re: 1. I am going to check with my colleague who uses yahoo to see if he can replicate the alert. He is not at all technical.
Re: 2. I am continuing to research this. I don't understand what SME is telling me.