Policy Enforcement
The MSA must check that the submitted mail is syntactically valid and conforms to the relevant site policies. RFC 6409 contains some optional features:
- Enforce submission rights guarantees that the envelope sender address is valid and authorized with the used authentication. This in essence complies with the SPF model specified in RFC 4408.
- May add sender permits to add a Sender address header field if the envelope sender address does not match any author address in the "From" header field. This roughly complies with the Sender ID model specified in RFC 4406 - ignoring the tricky case of Resent-From header fields not covered in RFC 6409.
Read more about this topic: Mail Submission Agent, Protocol
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—Noam Chomsky (b. 1928)