Email Workers auth requirement

[edevil]

Update docs mentioning that mail authentication will be required in the near future to be able to deliver email to CF.

Summary

Updated devdocs and added changelog entry.

Documentation checklist

• The documentation style guide has been adhered to.

[hyperlint-ai]

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PR Change Summary

Updated documentation to reflect upcoming mail authentication requirements for email delivery to Cloudflare.

• Added a section on mail authentication requirements effective from July 1, 2025.
• Updated DMARC enforcement recommendations for email senders.
• Clarified the necessity of SPF and DKIM for email delivery.

Modified Files

• src/content/docs/email-routing/postmaster.mdx

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[edevil]

Addressed all comments.