diff --git a/src/chat/chat_tests.rs b/src/chat/chat_tests.rs
index a82cd6305..57821f6a8 100644
--- a/src/chat/chat_tests.rs
+++ b/src/chat/chat_tests.rs
@@ -2632,12 +2632,6 @@ async fn test_can_send_group() -> Result<()> {
 /// the recipients can't see the identity of their fellow recipients.
 #[tokio::test(flavor = "multi_thread", worker_threads = 2)]
 async fn test_broadcast_members_cant_see_each_other() -> Result<()> {
-    fn contains(parsed: &MimeMessage, s: &str) -> bool {
-        assert_eq!(parsed.decrypting_failed, false);
-        let decoded_str = std::str::from_utf8(&parsed.decoded_data).unwrap();
-        decoded_str.contains(s)
-    }
-
     let mut tcm = TestContextManager::new();
     let alice = &tcm.alice().await;
     let bob = &tcm.bob().await;
@@ -2669,8 +2663,8 @@ async fn test_broadcast_members_cant_see_each_other() -> Result<()> {
         );
         let parsed = charlie.parse_msg(&auth_required).await;
         assert!(parsed.get_header(HeaderDef::AutocryptGossip).is_some());
-        assert!(contains(&parsed, "charlie@example.net"));
-        assert_eq!(contains(&parsed, "bob@example.net"), false);
+        assert!(parsed.decoded_data_contains("charlie@example.net"));
+        assert_eq!(parsed.decoded_data_contains("bob@example.net"), false);
 
         let parsed_by_bob = bob.parse_msg(&auth_required).await;
         assert!(parsed_by_bob.decrypting_failed);
@@ -2698,8 +2692,8 @@ async fn test_broadcast_members_cant_see_each_other() -> Result<()> {
         );
         let parsed = charlie.parse_msg(&member_added).await;
         assert!(parsed.get_header(HeaderDef::AutocryptGossip).is_some());
-        assert!(contains(&parsed, "charlie@example.net"));
-        assert_eq!(contains(&parsed, "bob@example.net"), false);
+        assert!(parsed.decoded_data_contains("charlie@example.net"));
+        assert_eq!(parsed.decoded_data_contains("bob@example.net"), false);
 
         let parsed_by_bob = bob.parse_msg(&member_added).await;
         assert!(parsed_by_bob.decrypting_failed);
@@ -2713,8 +2707,8 @@ async fn test_broadcast_members_cant_see_each_other() -> Result<()> {
         let hi_msg = alice.send_text(alice_broadcast_id, "hi").await;
         let parsed = charlie.parse_msg(&hi_msg).await;
         assert_eq!(parsed.header_exists(HeaderDef::AutocryptGossip), false);
-        assert_eq!(contains(&parsed, "charlie@example.net"), false);
-        assert_eq!(contains(&parsed, "bob@example.net"), false);
+        assert_eq!(parsed.decoded_data_contains("charlie@example.net"), false);
+        assert_eq!(parsed.decoded_data_contains("bob@example.net"), false);
 
         let parsed_by_bob = bob.parse_msg(&hi_msg).await;
         assert_eq!(parsed_by_bob.decrypting_failed, false);
@@ -2730,8 +2724,8 @@ async fn test_broadcast_members_cant_see_each_other() -> Result<()> {
             "charlie@example.net alice@example.org"
         );
         let parsed = charlie.parse_msg(&member_removed).await;
-        assert!(contains(&parsed, "charlie@example.net"));
-        assert_eq!(contains(&parsed, "bob@example.net"), false);
+        assert!(parsed.decoded_data_contains("charlie@example.net"));
+        assert_eq!(parsed.decoded_data_contains("bob@example.net"), false);
 
         let parsed_by_bob = bob.parse_msg(&member_removed).await;
         assert!(parsed_by_bob.decrypting_failed);
diff --git a/src/headerdef.rs b/src/headerdef.rs
index 32c2281b5..c57f05033 100644
--- a/src/headerdef.rs
+++ b/src/headerdef.rs
@@ -138,6 +138,9 @@ pub enum HeaderDef {
     /// Advertised gossip topic for one webxdc.
     IrohGossipTopic,
 
+    /// See <https://www.rfc-editor.org/rfc/rfc9788.html#name-hp-outer-header-field>.
+    HpOuter,
+
     #[cfg(test)]
     TestHeader,
 }
diff --git a/src/mimefactory/mimefactory_tests.rs b/src/mimefactory/mimefactory_tests.rs
index ce8b12c87..da49904da 100644
--- a/src/mimefactory/mimefactory_tests.rs
+++ b/src/mimefactory/mimefactory_tests.rs
@@ -837,6 +837,30 @@ async fn test_protected_headers_directive() -> Result<()> {
     Ok(())
 }
 
+#[tokio::test(flavor = "multi_thread", worker_threads = 2)]
+async fn test_hp_outer_headers() -> Result<()> {
+    let mut tcm = TestContextManager::new();
+    let t = &tcm.alice().await;
+    let chat_id = t.get_self_chat().await.id;
+
+    for std_hp_composing in [false, true] {
+        t.set_config_bool(Config::StdHeaderProtectionComposing, std_hp_composing)
+            .await?;
+        chat::send_text_msg(t, chat_id, "hi!".to_string()).await?;
+        let sent_msg = t.pop_sent_msg().await;
+        let msg = MimeMessage::from_bytes(t, sent_msg.payload.as_bytes(), None).await?;
+        assert_eq!(msg.header_exists(HeaderDef::HpOuter), std_hp_composing);
+        for hdr in ["Date", "From", "Message-ID"] {
+            assert_eq!(
+                msg.decoded_data_contains(&format!("HP-Outer: {hdr}:")),
+                std_hp_composing,
+            );
+        }
+        assert!(!msg.decoded_data_contains("HP-Outer: Content-Type"));
+    }
+    Ok(())
+}
+
 #[tokio::test(flavor = "multi_thread", worker_threads = 2)]
 async fn test_dont_remove_self() -> Result<()> {
     let mut tcm = TestContextManager::new();
diff --git a/src/mimeparser.rs b/src/mimeparser.rs
index 703b16469..4b50ca2e1 100644
--- a/src/mimeparser.rs
+++ b/src/mimeparser.rs
@@ -1007,6 +1007,14 @@ impl MimeMessage {
         self.headers.contains_key(hname) || self.headers_removed.contains(hname)
     }
 
+    #[cfg(test)]
+    /// Returns whether the decrypted data contains the given `&str`.
+    pub(crate) fn decoded_data_contains(&self, s: &str) -> bool {
+        assert!(!self.decrypting_failed);
+        let decoded_str = str::from_utf8(&self.decoded_data).unwrap();
+        decoded_str.contains(s)
+    }
+
     /// Returns `Chat-Group-ID` header value if it is a valid group ID.
     pub fn get_chat_group_id(&self) -> Option<&str> {
         self.get_header(HeaderDef::ChatGroupId)