Determining Yahoo's mail server infrastructure is a complex task due to frequent changes, geographic distribution, and a lack of publicly available server details. Yahoo Business Mail users should utilize 'mx1.biz.mail.yahoo.com' and 'mx2.biz.mail.yahoo.com' with priority 10. While 'smtp.mail.yahoo.com' is often cited for outgoing mail, exact settings must be verified. Experts advise against relying on 'gm' or 'am' domains or IP ranges. DNS records, tools like nslookup and dig, and monitoring Yahoo's postmaster site are highly recommended. Authentication (SPF, DKIM, DMARC), domain reputation, and engagement metrics are crucial for deliverability. The AOL and Yahoo postmaster tools are considered the most reliable way to understand the infrastructure.