SPF, DMARC, DKIM is setup correctly. Domain name A/AAA/TXT/MX records all setup and propagated throughout the world. Mail server tested with external tools like mail-tester.com
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But there is a whole new world of “other mail servers” now. Some mail servers are okay. Delivery works fine. Some mail servers have odd policies and implement strict block lists and maintain their own rep of each domain/sender.