Gmail's rules (from 1 February 2024, for senders of more than 5,000 messages a day to Gmail): SPF and DKIM, a DMARC record (policy none is enough) with the From domain aligned to SPF or DKIM, valid forward and reverse DNS, TLS, RFC 5322-compliant messages, a spam rate in Postmaster Tools below 0.3% (ideally below 0.1%) and one-click unsubscribe for marketing mail using List-Unsubscribe and List-Unsubscribe-Post (RFC 8058), honored within two days. Yahoo published equivalent rules.
Microsoft applies the same authentication baseline to Outlook.com, Hotmail and Live addresses from 5 May 2025 for high-volume senders and rejects mail that does not meet it. Non-compliance shows up as temporary and then permanent SMTP rejections citing authentication, so check the receiving provider's error text rather than guessing.