Authenticate the sending domain
Mail providers use DNS records such as SPF and DKIM to evaluate whether a sender is authorized and whether a message has a verifiable signature. Follow the sending provider's exact record values; do not copy generic examples into production.
If you also use DMARC, publish a policy that matches your monitoring and enforcement plan. A strict policy introduced before every legitimate sender is accounted for can cause valid mail to be rejected or quarantined.
Test with real receiving systems
Send a small test to mailboxes you can inspect at different providers. Check authentication results, message headers, bounce events, and spam placement rather than treating the send step alone as proof of inbox delivery.
Inventory all systems that send as your domain, including applications and workflows, before changing policy. Roll out stricter enforcement gradually and monitor reports so a record change does not disrupt legitimate business mail.