diff --git a/crates/nuncio-filter/src/engine.rs b/crates/nuncio-filter/src/engine.rs
index e3f4822..bce77e8 100644
--- a/crates/nuncio-filter/src/engine.rs
+++ b/crates/nuncio-filter/src/engine.rs
@@ -97,9 +97,12 @@ impl CompiledFilter {
                     .unwrap_or("");
                 Self::eval_string_op(body, &leaf.operator, &leaf.value, regexes)
             }
-            FilterField::Folder | FilterField::Account => {
+            FilterField::Folder => {
                 Self::eval_string_op(&email.folder_id, &leaf.operator, &leaf.value, regexes)
             }
+            FilterField::Account => {
+                Self::eval_string_op(&email.account_id, &leaf.operator, &leaf.value, regexes)
+            }
             FilterField::HasAttachment => {
                 let has = !email.attachments.is_empty();
                 if let FilterValue::Boolean(b) = leaf.value {
@@ -148,7 +151,11 @@ impl CompiledFilter {
                 }
             }
             FilterField::Header(_name) => {
-                // Header evaluation fallback
+                // Unreachable defensive branch: `NsqlValidator::pass1_field_types`
+                // rejects any rule with a `header[...]` condition before it can
+                // reach a `CompiledFilter`, since `Email` has no parsed-headers
+                // field to match against. Kept only so this match stays
+                // exhaustive over `FilterField`.
                 false
             }
         }
@@ -475,6 +482,30 @@ mod tests {
         assert_eq!(engine.evaluate(&email).len(), 1);
     }
 
+    #[test]
+    fn test_account_condition_field_matches_account_id_not_folder_id() {
+        let nsql = "WHERE account = 'acct-a' ACTION MARK READ";
+        let rule = crate::parser::NsqlParser::parse_rule("Account Filter", 1, nsql).unwrap();
+        let engine = FilterEngine::new(vec![rule]).unwrap();
+
+        // Same folder_id, different account_id: a rule matching on the
+        // `account` condition field must key off email.account_id, never
+        // fall through to comparing folder_id.
+        let email_a = test_email("acct-a", "Anything", "shared-folder");
+        assert_eq!(
+            engine.evaluate(&email_a).len(),
+            1,
+            "rule on account = 'acct-a' must match a message with account_id 'acct-a'"
+        );
+
+        let email_b = test_email("acct-b", "Anything", "shared-folder");
+        assert!(
+            engine.evaluate(&email_b).is_empty(),
+            "rule on account = 'acct-a' must not match a message with account_id 'acct-b', \
+             even when it shares the same folder_id as an acct-a message"
+        );
+    }
+
     #[test]
     fn test_not_in_operator_is_complement_of_in() {
         let nsql = "WHERE folder NOT IN ('spam', 'trash') ACTION MARK READ";
diff --git a/crates/nuncio-filter/src/validator.rs b/crates/nuncio-filter/src/validator.rs
index 145d836..ff67cbd 100644
--- a/crates/nuncio-filter/src/validator.rs
+++ b/crates/nuncio-filter/src/validator.rs
@@ -83,11 +83,14 @@ impl NsqlValidator {
                         "field 'has_attachment' requires a boolean value".to_string(),
                     )),
                 },
+                FilterField::Header(_) => Err(ValidationError::FieldTypeMismatch(
+                    "header field conditions are not yet supported: message headers are not indexed"
+                        .to_string(),
+                )),
                 FilterField::Subject
                 | FilterField::From
                 | FilterField::To
                 | FilterField::Body
-                | FilterField::Header(_)
                 | FilterField::Folder
                 | FilterField::Account => match &leaf.value {
                     FilterValue::String(_) | FilterValue::List(_) => Ok(()),
@@ -322,6 +325,18 @@ mod tests {
         assert!(matches!(err, ValidationError::FieldTypeMismatch(_)));
     }
 
+    #[test]
+    fn test_pass1_rejects_unsupported_header_field_conditions() {
+        let rule = crate::parser::NsqlParser::parse_rule(
+            "Header Spam Score",
+            1,
+            "WHERE header['X-Spam-Score'] = '10' ACTION MARK READ",
+        )
+        .unwrap();
+        let err = NsqlValidator::validate(&rule, &ValidationOptions::default()).unwrap_err();
+        assert!(matches!(err, ValidationError::FieldTypeMismatch(_)));
+    }
+
     #[test]
     fn test_pass4_redos_validation() {
         let rule = crate::parser::NsqlParser::parse_rule(