diff --git a/.github/blunderbuss.yml b/.github/blunderbuss.yml
index 44ab091e1..97998e463 100644
--- a/.github/blunderbuss.yml
+++ b/.github/blunderbuss.yml
@@ -1,9 +1,9 @@
 # Configuration for the Blunderbuss GitHub app. For more info see
 # https://github.com/googleapis/repo-automation-bots/tree/main/packages/blunderbuss
 assign_issues:
-  - maitrimangal
+  - michaelpri10
 assign_prs:
-  - maitrimangal
+  - michaelpri10
 assign_prs_by:
 - labels:
   - samples
diff --git a/CHANGELOG.md b/CHANGELOG.md
index 8721be429..92946f184 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,5 +1,12 @@
 # Changelog
 
+## [1.126.4](https://github.com/googleapis/java-pubsub/compare/v1.126.3...v1.126.4) (2024-02-09)
+
+
+### Bug Fixes
+
+* Message ordering fix for [#1889](https://github.com/googleapis/java-pubsub/issues/1889) ([#1903](https://github.com/googleapis/java-pubsub/issues/1903)) ([22a87c6](https://github.com/googleapis/java-pubsub/commit/22a87c67f07b55266e277f83f5ceb17d9f32f67e))
+
 ## [1.126.3](https://github.com/googleapis/java-pubsub/compare/v1.126.2...v1.126.3) (2024-02-08)
 
 
diff --git a/README.md b/README.md
index 68423c089..89680132d 100644
--- a/README.md
+++ b/README.md
@@ -44,7 +44,7 @@ If you are using Maven without the BOM, add this to your dependencies:
 <dependency>
   <groupId>com.google.cloud</groupId>
   <artifactId>google-cloud-pubsub</artifactId>
-  <version>1.126.2</version>
+  <version>1.126.3</version>
 </dependency>
 
 ```
@@ -59,13 +59,13 @@ implementation 'com.google.cloud:google-cloud-pubsub'
 If you are using Gradle without BOM, add this to your dependencies:
 
 ```Groovy
-implementation 'com.google.cloud:google-cloud-pubsub:1.126.2'
+implementation 'com.google.cloud:google-cloud-pubsub:1.126.3'
 ```
 
 If you are using SBT, add this to your dependencies:
 
 ```Scala
-libraryDependencies += "com.google.cloud" % "google-cloud-pubsub" % "1.126.2"
+libraryDependencies += "com.google.cloud" % "google-cloud-pubsub" % "1.126.3"
 ```
 <!-- {x-version-update-end} -->
 
@@ -409,7 +409,7 @@ Java is a registered trademark of Oracle and/or its affiliates.
 [kokoro-badge-link-5]: http://storage.googleapis.com/cloud-devrel-public/java/badges/java-pubsub/java11.html
 [stability-image]: https://img.shields.io/badge/stability-stable-green
 [maven-version-image]: https://img.shields.io/maven-central/v/com.google.cloud/google-cloud-pubsub.svg
-[maven-version-link]: https://central.sonatype.com/artifact/com.google.cloud/google-cloud-pubsub/1.126.2
+[maven-version-link]: https://central.sonatype.com/artifact/com.google.cloud/google-cloud-pubsub/1.126.3
 [authentication]: https://github.com/googleapis/google-cloud-java#authentication
 [auth-scopes]: https://developers.google.com/identity/protocols/oauth2/scopes
 [predefined-iam-roles]: https://cloud.google.com/iam/docs/understanding-roles#predefined_roles
diff --git a/google-cloud-pubsub-bom/pom.xml b/google-cloud-pubsub-bom/pom.xml
index cbf0b858c..795bf000c 100644
--- a/google-cloud-pubsub-bom/pom.xml
+++ b/google-cloud-pubsub-bom/pom.xml
@@ -3,7 +3,7 @@
   <modelVersion>4.0.0</modelVersion>
   <groupId>com.google.cloud</groupId>
   <artifactId>google-cloud-pubsub-bom</artifactId>
-  <version>1.126.3</version><!-- {x-version-update:google-cloud-pubsub:current} -->
+  <version>1.126.4</version><!-- {x-version-update:google-cloud-pubsub:current} -->
   <packaging>pom</packaging>
   <parent>
     <groupId>com.google.cloud</groupId>
@@ -52,17 +52,17 @@
       <dependency>
         <groupId>com.google.cloud</groupId>
         <artifactId>google-cloud-pubsub</artifactId>
-        <version>1.126.3</version><!-- {x-version-update:google-cloud-pubsub:current} -->
+        <version>1.126.4</version><!-- {x-version-update:google-cloud-pubsub:current} -->
       </dependency>
       <dependency>
         <groupId>com.google.api.grpc</groupId>
         <artifactId>grpc-google-cloud-pubsub-v1</artifactId>
-        <version>1.108.3</version><!-- {x-version-update:grpc-google-cloud-pubsub-v1:current} -->
+        <version>1.108.4</version><!-- {x-version-update:grpc-google-cloud-pubsub-v1:current} -->
       </dependency>
       <dependency>
         <groupId>com.google.api.grpc</groupId>
         <artifactId>proto-google-cloud-pubsub-v1</artifactId>
-        <version>1.108.3</version><!-- {x-version-update:proto-google-cloud-pubsub-v1:current} -->
+        <version>1.108.4</version><!-- {x-version-update:proto-google-cloud-pubsub-v1:current} -->
       </dependency>
     </dependencies>
   </dependencyManagement>
diff --git a/google-cloud-pubsub/pom.xml b/google-cloud-pubsub/pom.xml
index 650cb553f..cb705395d 100644
--- a/google-cloud-pubsub/pom.xml
+++ b/google-cloud-pubsub/pom.xml
@@ -3,7 +3,7 @@
   <modelVersion>4.0.0</modelVersion>
   <groupId>com.google.cloud</groupId>
   <artifactId>google-cloud-pubsub</artifactId>
-  <version>1.126.3</version><!-- {x-version-update:google-cloud-pubsub:current} -->
+  <version>1.126.4</version><!-- {x-version-update:google-cloud-pubsub:current} -->
   <packaging>jar</packaging>
   <name>Google Cloud Pub/Sub</name>
   <url>https://github.com/googleapis/java-pubsub</url>
@@ -11,7 +11,7 @@
   <parent>
     <groupId>com.google.cloud</groupId>
     <artifactId>google-cloud-pubsub-parent</artifactId>
-    <version>1.126.3</version><!-- {x-version-update:google-cloud-pubsub:current} -->
+    <version>1.126.4</version><!-- {x-version-update:google-cloud-pubsub:current} -->
   </parent>
   <properties>
     <site.installationModule>google-cloud-pubsub</site.installationModule>
diff --git a/google-cloud-pubsub/src/main/java/com/google/cloud/pubsub/v1/MessageDispatcher.java b/google-cloud-pubsub/src/main/java/com/google/cloud/pubsub/v1/MessageDispatcher.java
index 635bc92d5..1810badd2 100644
--- a/google-cloud-pubsub/src/main/java/com/google/cloud/pubsub/v1/MessageDispatcher.java
+++ b/google-cloud-pubsub/src/main/java/com/google/cloud/pubsub/v1/MessageDispatcher.java
@@ -31,6 +31,7 @@
 import java.util.ArrayList;
 import java.util.HashMap;
 import java.util.Iterator;
+import java.util.LinkedHashMap;
 import java.util.List;
 import java.util.Map;
 import java.util.Map.Entry;
@@ -92,8 +93,8 @@ class MessageDispatcher {
   private final LinkedBlockingQueue<AckRequestData> pendingAcks = new LinkedBlockingQueue<>();
   private final LinkedBlockingQueue<AckRequestData> pendingNacks = new LinkedBlockingQueue<>();
   private final LinkedBlockingQueue<AckRequestData> pendingReceipts = new LinkedBlockingQueue<>();
-  private final ConcurrentMap<String, ReceiptCompleteData> outstandingReceipts =
-      new ConcurrentHashMap<String, ReceiptCompleteData>();
+  private final LinkedHashMap<String, ReceiptCompleteData> outstandingReceipts =
+      new LinkedHashMap<String, ReceiptCompleteData>();
   private final AtomicInteger messageDeadlineSeconds = new AtomicInteger();
   private final AtomicBoolean extendDeadline = new AtomicBoolean(true);
   private final Lock jobLock;
@@ -397,7 +398,9 @@ void processReceivedMessages(List<ReceivedMessage> messages) {
       if (this.exactlyOnceDeliveryEnabled.get()) {
         // For exactly once deliveries we don't add to outstanding batch because we first
         // process the receipt modack. If that is successful then we process the message.
-        outstandingReceipts.put(message.getAckId(), new ReceiptCompleteData(outstandingMessage));
+        synchronized (outstandingReceipts) {
+          outstandingReceipts.put(message.getAckId(), new ReceiptCompleteData(outstandingMessage));
+        }
       } else if (pendingMessages.putIfAbsent(message.getAckId(), ackHandler) != null) {
         // putIfAbsent puts ackHandler if ackID isn't previously mapped, then return the
         // previously-mapped element.
@@ -417,33 +420,36 @@ void processReceivedMessages(List<ReceivedMessage> messages) {
   }
 
   void notifyAckSuccess(AckRequestData ackRequestData) {
-
-    if (outstandingReceipts.containsKey(ackRequestData.getAckId())) {
-      outstandingReceipts.get(ackRequestData.getAckId()).notifyReceiptComplete();
-      List<OutstandingMessage> outstandingBatch = new ArrayList<>();
-
-      for (Iterator<Entry<String, ReceiptCompleteData>> it =
-              outstandingReceipts.entrySet().iterator();
-          it.hasNext(); ) {
-        Map.Entry<String, ReceiptCompleteData> receipt = it.next();
-        // If receipt is complete then add to outstandingBatch to process the batch
-        if (receipt.getValue().isReceiptComplete()) {
-          it.remove();
-          if (pendingMessages.putIfAbsent(
-                  receipt.getKey(), receipt.getValue().getOutstandingMessage().ackHandler)
-              == null) {
-            outstandingBatch.add(receipt.getValue().getOutstandingMessage());
+    synchronized (outstandingReceipts) {
+      if (outstandingReceipts.containsKey(ackRequestData.getAckId())) {
+        outstandingReceipts.get(ackRequestData.getAckId()).notifyReceiptComplete();
+        List<OutstandingMessage> outstandingBatch = new ArrayList<>();
+
+        for (Iterator<Entry<String, ReceiptCompleteData>> it =
+                outstandingReceipts.entrySet().iterator();
+            it.hasNext(); ) {
+          Map.Entry<String, ReceiptCompleteData> receipt = it.next();
+          // If receipt is complete then add to outstandingBatch to process the batch
+          if (receipt.getValue().isReceiptComplete()) {
+            it.remove();
+            if (pendingMessages.putIfAbsent(
+                    receipt.getKey(), receipt.getValue().getOutstandingMessage().ackHandler)
+                == null) {
+              outstandingBatch.add(receipt.getValue().getOutstandingMessage());
+            }
+          } else {
+            break;
           }
-        } else {
-          break;
         }
+        processBatch(outstandingBatch);
       }
-      processBatch(outstandingBatch);
     }
   }
 
   void notifyAckFailed(AckRequestData ackRequestData) {
-    outstandingReceipts.remove(ackRequestData.getAckId());
+    synchronized (outstandingReceipts) {
+      outstandingReceipts.remove(ackRequestData.getAckId());
+    }
   }
 
   private void processBatch(List<OutstandingMessage> batch) {
diff --git a/grpc-google-cloud-pubsub-v1/pom.xml b/grpc-google-cloud-pubsub-v1/pom.xml
index 236f1fbe5..ab59fe1ad 100644
--- a/grpc-google-cloud-pubsub-v1/pom.xml
+++ b/grpc-google-cloud-pubsub-v1/pom.xml
@@ -4,13 +4,13 @@
   <modelVersion>4.0.0</modelVersion>
   <groupId>com.google.api.grpc</groupId>
   <artifactId>grpc-google-cloud-pubsub-v1</artifactId>
-  <version>1.108.3</version><!-- {x-version-update:grpc-google-cloud-pubsub-v1:current} -->
+  <version>1.108.4</version><!-- {x-version-update:grpc-google-cloud-pubsub-v1:current} -->
   <name>grpc-google-cloud-pubsub-v1</name>
   <description>GRPC library for grpc-google-cloud-pubsub-v1</description>
   <parent>
     <groupId>com.google.cloud</groupId>
     <artifactId>google-cloud-pubsub-parent</artifactId>
-    <version>1.126.3</version><!-- {x-version-update:google-cloud-pubsub:current} -->
+    <version>1.126.4</version><!-- {x-version-update:google-cloud-pubsub:current} -->
   </parent>
   <dependencies>
     <dependency>
diff --git a/pom.xml b/pom.xml
index 321bef25a..6de969e6a 100644
--- a/pom.xml
+++ b/pom.xml
@@ -4,7 +4,7 @@
   <groupId>com.google.cloud</groupId>
   <artifactId>google-cloud-pubsub-parent</artifactId>
   <packaging>pom</packaging>
-  <version>1.126.3</version><!-- {x-version-update:google-cloud-pubsub:current} -->
+  <version>1.126.4</version><!-- {x-version-update:google-cloud-pubsub:current} -->
   <name>Google Cloud Pub/Sub Parent</name>
   <url>https://github.com/googleapis/java-pubsub</url>
   <description>
@@ -69,17 +69,17 @@
       <dependency>
         <groupId>com.google.api.grpc</groupId>
         <artifactId>proto-google-cloud-pubsub-v1</artifactId>
-        <version>1.108.3</version><!-- {x-version-update:proto-google-cloud-pubsub-v1:current} -->
+        <version>1.108.4</version><!-- {x-version-update:proto-google-cloud-pubsub-v1:current} -->
       </dependency>
       <dependency>
         <groupId>com.google.api.grpc</groupId>
         <artifactId>grpc-google-cloud-pubsub-v1</artifactId>
-        <version>1.108.3</version><!-- {x-version-update:grpc-google-cloud-pubsub-v1:current} -->
+        <version>1.108.4</version><!-- {x-version-update:grpc-google-cloud-pubsub-v1:current} -->
       </dependency>
       <dependency>
         <groupId>com.google.cloud</groupId>
         <artifactId>google-cloud-pubsub</artifactId>
-        <version>1.126.3</version><!-- {x-version-update:google-cloud-pubsub:current} -->
+        <version>1.126.4</version><!-- {x-version-update:google-cloud-pubsub:current} -->
       </dependency>
 
       <!-- Test dependencies -->
diff --git a/proto-google-cloud-pubsub-v1/pom.xml b/proto-google-cloud-pubsub-v1/pom.xml
index 6e94b2ee1..e06710a62 100644
--- a/proto-google-cloud-pubsub-v1/pom.xml
+++ b/proto-google-cloud-pubsub-v1/pom.xml
@@ -4,13 +4,13 @@
   <modelVersion>4.0.0</modelVersion>
   <groupId>com.google.api.grpc</groupId>
   <artifactId>proto-google-cloud-pubsub-v1</artifactId>
-  <version>1.108.3</version><!-- {x-version-update:proto-google-cloud-pubsub-v1:current} -->
+  <version>1.108.4</version><!-- {x-version-update:proto-google-cloud-pubsub-v1:current} -->
   <name>proto-google-cloud-pubsub-v1</name>
   <description>PROTO library for proto-google-cloud-pubsub-v1</description>
   <parent>
     <groupId>com.google.cloud</groupId>
     <artifactId>google-cloud-pubsub-parent</artifactId>
-    <version>1.126.3</version><!-- {x-version-update:google-cloud-pubsub:current} -->
+    <version>1.126.4</version><!-- {x-version-update:google-cloud-pubsub:current} -->
   </parent>
   <dependencies>
     <dependency>
diff --git a/samples/install-without-bom/pom.xml b/samples/install-without-bom/pom.xml
index 8a23573ba..b6169a62c 100644
--- a/samples/install-without-bom/pom.xml
+++ b/samples/install-without-bom/pom.xml
@@ -44,7 +44,7 @@
     <dependency>
       <groupId>com.google.cloud</groupId>
       <artifactId>google-cloud-pubsub</artifactId>
-      <version>1.126.2</version>
+      <version>1.126.3</version>
     </dependency>
     <!-- [END pubsub_install_without_bom] -->
 
diff --git a/samples/snapshot/pom.xml b/samples/snapshot/pom.xml
index b1e557e8b..adcd70028 100644
--- a/samples/snapshot/pom.xml
+++ b/samples/snapshot/pom.xml
@@ -43,7 +43,7 @@
     <dependency>
       <groupId>com.google.cloud</groupId>
       <artifactId>google-cloud-pubsub</artifactId>
-      <version>1.126.3</version>
+      <version>1.126.4</version>
     </dependency>
     <!-- {x-version-update-end} -->
 
diff --git a/versions.txt b/versions.txt
index 8cce4bec8..451bc8f07 100644
--- a/versions.txt
+++ b/versions.txt
@@ -1,6 +1,6 @@
 # Format:
 # module:released-version:current-version
 
-google-cloud-pubsub:1.126.3:1.126.3
-grpc-google-cloud-pubsub-v1:1.108.3:1.108.3
-proto-google-cloud-pubsub-v1:1.108.3:1.108.3
+google-cloud-pubsub:1.126.4:1.126.4
+grpc-google-cloud-pubsub-v1:1.108.4:1.108.4
+proto-google-cloud-pubsub-v1:1.108.4:1.108.4