Looking for Yahoo eMail contact

Hello, I’m looking for a Yahoo email administrator who could contact me offlist. I have a customer with a clean record that is getting thsi error: Error: "421 4.7.1 [TS03] All messages from x.x.x.x permanently deferred" when sending email to Yahoo The customer is a local non-profit and sends a very limited amount of emails to members, suppliers and other contacts. Mailing-lists are only used to contact members of the NPO. I checked the recommendation listed at https://help.yahoo.com/kb/postmaster/SLN3436.html and checked his IP address on several “multi-rbl” lookup sites. All looks clean. So I need more input to understand what we need to correct. Thank you very much in advance. Best regards, Marc