Define governed secure mail profiles

docs/MAIL_PROTOCOL_ROADMAP.md

@@ -13,6 +13,28 @@ and JMAP are deferred until the IMAP mailbox MVP is stable.
 This baseline matches the first production use case: send campaign mail, append
 sent copies when configured, and inspect mailboxes read-only.
 
+## Message Protection Profiles
+
+The product and security profile approved on 2026-08-04 makes S/MIME the first
+institutional signing/encryption profile and OpenPGP an additional explicit
+profile. Neither profile is implemented by treating protection as a local Mail
+toggle:
+
+- private-key custody belongs to an Encryption/KMS provider and usable private
+  keys are never persisted by Mail;
+- recipient certificates and keys initially come from administered directory
+  or LDAP sources; opportunistic Internet discovery is deferred;
+- required signing or encryption fails closed when material is missing,
+  expired, revoked, unverifiable, or its provider is unavailable;
+- plaintext fallback is permitted only by an explicit, audited policy and is
+  never inferred from provider failure; and
+- delivery evidence pins the signing identity, trust/revocation evidence,
+  algorithm suite, key version, and any explicit downgrade decision.
+
+Provider-neutral S/MIME custody and interoperability fixtures are the first
+implementation slice. OpenPGP uses the same no-silent-downgrade boundary after
+the S/MIME profile is stable.
+
 ## JMAP
 
 JMAP is the preferred future sync/search protocol where target mail servers