When I tried to reply to an email from a friend and cc my partner, who also has yahoo mail, yahoo blocked my message! I got error code "554: Message not allowed - [PH01] Email not accepted for policy reasons. Please visit https://postmaster.yahooinc.com/error-codes" This was just a reply in a message chain about getting together for dinner. It didn't look like spam in any way. Looks like yahoo needs to fix whatever "policy reasons" lead it to randomly reject personal emails.