diff --git a/CHANGELOG.md b/CHANGELOG.md
index 605e5f59..de1a36dc 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -38,6 +38,7 @@ If you are upgrading from v0.16.x, replace the binary (or run `docker pull`). If
   - `VacationResponse/set`: incorrect singleton handling.
 - OIDC: Add default domain name to groups that are not email addresses.
 - RocksDB: Enable blob garbage collection to reclaim disk space from deleted blobs.
+- Sieve: `include` statements ignore capitalisation of sub-script names (#1643)
 
 ## [0.16.9] - 2026-06-15
 
diff --git a/crates/email/src/sieve/ingest.rs b/crates/email/src/sieve/ingest.rs
index 6d85bb4d..80f36fcb 100644
--- a/crates/email/src/sieve/ingest.rs
+++ b/crates/email/src/sieve/ingest.rs
@@ -611,7 +611,7 @@ impl SieveScriptIngest for Server {
                 account_id,
                 Collection::SieveScript,
                 SieveField::Name,
-                name.as_bytes(),
+                name.to_lowercase().as_bytes(),
             )
             .await
             .caused_by(trc::location!())?
diff --git a/tests/resources/jmap/sieve/test_include_case.sieve b/tests/resources/jmap/sieve/test_include_case.sieve
new file mode 100644
index 00000000..27c10c01
--- /dev/null
+++ b/tests/resources/jmap/sieve/test_include_case.sieve
@@ -0,0 +1,5 @@
+require ["include", "ihave"];
+
+include :personal "Test Script";
+
+error "'stop' within included script ignored or mixed-case include failed.";
diff --git a/tests/src/jmap/mail/sieve_script.rs b/tests/src/jmap/mail/sieve_script.rs
index bb20681f..ff232899 100644
--- a/tests/src/jmap/mail/sieve_script.rs
+++ b/tests/src/jmap/mail/sieve_script.rs
@@ -339,6 +339,49 @@ pub async fn test(test: &TestServer) {
     )
     .await;
 
+    client
+        .sieve_script_create(
+            "Test Script",
+            concat!(
+                "require \"reject\";\n",
+                "reject \"Rejected from a mixed-case included script.\";\n",
+                "stop;\n"
+            )
+            .as_bytes()
+            .to_vec(),
+            false,
+        )
+        .await
+        .unwrap();
+    client
+        .sieve_script_create("test_include_case", get_script("test_include_case"), true)
+        .await
+        .unwrap();
+    lmtp.ingest(
+        "bill@remote.org",
+        &["jdoe@example.com"],
+        concat!(
+            "From: bill@remote.org\r\n",
+            "Bcc: Undisclosed recipients;\r\n",
+            "Message-ID: <5678@example.com>\r\n",
+            "Subject: Holidays\r\n",
+            "\r\n",
+            "Remember to file your T.P.S. reports before ",
+            "going on holidays."
+        ),
+    )
+    .await;
+
+    assert_message_delivery(
+        &mut smtp_rx,
+        MockMessage::new(
+            "<>",
+            ["<bill@remote.org>"],
+            "@Rejected from a mixed-case included script",
+        ),
+    )
+    .await;
+
     // Run include global tests
     client
         .sieve_script_create(