diff --git a/src/Symfony/Component/Mailer/Bridge/Mailchimp/Tests/Transport/MandrillApiTransportTest.php b/src/Symfony/Component/Mailer/Bridge/Mailchimp/Tests/Transport/MandrillApiTransportTest.php
index 51cfa45c0f1fa..152e198309dc3 100644
--- a/src/Symfony/Component/Mailer/Bridge/Mailchimp/Tests/Transport/MandrillApiTransportTest.php
+++ b/src/Symfony/Component/Mailer/Bridge/Mailchimp/Tests/Transport/MandrillApiTransportTest.php
@@ -148,6 +148,7 @@ public function testCanHaveMultipleTags()
     {
         $email = new Email();
         $email->getHeaders()->add(new TagHeader('password-reset,user'));
+        $email->getHeaders()->add(new TagHeader('another'));
         $envelope = new Envelope(new Address('alice@system.com'), [new Address('bob@system.com')]);
 
         $transport = new MandrillApiTransport('ACCESS_KEY');
@@ -158,6 +159,6 @@ public function testCanHaveMultipleTags()
         $this->assertArrayHasKey('message', $payload);
         $this->assertArrayNotHasKey('headers', $payload['message']);
         $this->assertArrayHasKey('tags', $payload['message']);
-        $this->assertSame(['password-reset', 'user'], $payload['message']['tags']);
+        $this->assertSame(['password-reset', 'user', 'another'], $payload['message']['tags']);
     }
 }
diff --git a/src/Symfony/Component/Mailer/Bridge/Mailchimp/Tests/Transport/MandrillHttpTransportTest.php b/src/Symfony/Component/Mailer/Bridge/Mailchimp/Tests/Transport/MandrillHttpTransportTest.php
index 48faa7f2fd2b5..8f5eeb1f02012 100644
--- a/src/Symfony/Component/Mailer/Bridge/Mailchimp/Tests/Transport/MandrillHttpTransportTest.php
+++ b/src/Symfony/Component/Mailer/Bridge/Mailchimp/Tests/Transport/MandrillHttpTransportTest.php
@@ -112,6 +112,7 @@ public function testTagAndMetadataHeaders()
         $email = new Email();
         $email->getHeaders()->addTextHeader('foo', 'bar');
         $email->getHeaders()->add(new TagHeader('password-reset,user'));
+        $email->getHeaders()->add(new TagHeader('another'));
         $email->getHeaders()->add(new MetadataHeader('Color', 'blue'));
         $email->getHeaders()->add(new MetadataHeader('Client-ID', '12345'));
 
@@ -122,7 +123,7 @@ public function testTagAndMetadataHeaders()
 
         $this->assertCount(3, $email->getHeaders()->toArray());
         $this->assertSame('foo: bar', $email->getHeaders()->get('FOO')->toString());
-        $this->assertSame('X-MC-Tags: password-reset,user', $email->getHeaders()->get('X-MC-Tags')->toString());
+        $this->assertSame('X-MC-Tags: password-reset,user,another', $email->getHeaders()->get('X-MC-Tags')->toString());
         $this->assertSame('X-MC-Metadata: '.json_encode(['Color' => 'blue', 'Client-ID' => '12345']), $email->getHeaders()->get('X-MC-Metadata')->toString());
     }
 }
diff --git a/src/Symfony/Component/Mailer/Bridge/Mailchimp/Tests/Transport/MandrillSmtpTransportTest.php b/src/Symfony/Component/Mailer/Bridge/Mailchimp/Tests/Transport/MandrillSmtpTransportTest.php
index c6ed7cd9888d5..e2b0e7ff38c7a 100644
--- a/src/Symfony/Component/Mailer/Bridge/Mailchimp/Tests/Transport/MandrillSmtpTransportTest.php
+++ b/src/Symfony/Component/Mailer/Bridge/Mailchimp/Tests/Transport/MandrillSmtpTransportTest.php
@@ -24,6 +24,7 @@ public function testTagAndMetadataHeaders()
         $email = new Email();
         $email->getHeaders()->addTextHeader('foo', 'bar');
         $email->getHeaders()->add(new TagHeader('password-reset,user'));
+        $email->getHeaders()->add(new TagHeader('another'));
         $email->getHeaders()->add(new MetadataHeader('Color', 'blue'));
         $email->getHeaders()->add(new MetadataHeader('Client-ID', '12345'));
 
@@ -34,7 +35,7 @@ public function testTagAndMetadataHeaders()
 
         $this->assertCount(3, $email->getHeaders()->toArray());
         $this->assertSame('foo: bar', $email->getHeaders()->get('FOO')->toString());
-        $this->assertSame('X-MC-Tags: password-reset,user', $email->getHeaders()->get('X-MC-Tags')->toString());
+        $this->assertSame('X-MC-Tags: password-reset,user,another', $email->getHeaders()->get('X-MC-Tags')->toString());
         $this->assertSame('X-MC-Metadata: '.json_encode(['Color' => 'blue', 'Client-ID' => '12345']), $email->getHeaders()->get('X-MC-Metadata')->toString());
     }
 }
diff --git a/src/Symfony/Component/Mailer/Bridge/Mailchimp/Transport/MandrillApiTransport.php b/src/Symfony/Component/Mailer/Bridge/Mailchimp/Transport/MandrillApiTransport.php
index 2ba4f5e3b56ef..2c0047025716c 100644
--- a/src/Symfony/Component/Mailer/Bridge/Mailchimp/Transport/MandrillApiTransport.php
+++ b/src/Symfony/Component/Mailer/Bridge/Mailchimp/Transport/MandrillApiTransport.php
@@ -124,7 +124,10 @@ private function getPayload(Email $email, Envelope $envelope): array
             }
 
             if ($header instanceof TagHeader) {
-                $payload['message']['tags'] = explode(',', $header->getValue());
+                $payload['message']['tags'] = array_merge(
+                    $payload['message']['tags'] ?? [],
+                    explode(',', $header->getValue())
+                );
 
                 continue;
             }
diff --git a/src/Symfony/Component/Mailer/Bridge/Mailchimp/Transport/MandrillHeadersTrait.php b/src/Symfony/Component/Mailer/Bridge/Mailchimp/Transport/MandrillHeadersTrait.php
index 2ca440560aeb2..73c3dbd010640 100644
--- a/src/Symfony/Component/Mailer/Bridge/Mailchimp/Transport/MandrillHeadersTrait.php
+++ b/src/Symfony/Component/Mailer/Bridge/Mailchimp/Transport/MandrillHeadersTrait.php
@@ -36,10 +36,11 @@ private function addMandrillHeaders(Message $message): void
     {
         $headers = $message->getHeaders();
         $metadata = [];
+        $tags = [];
 
         foreach ($headers->all() as $name => $header) {
             if ($header instanceof TagHeader) {
-                $headers->addTextHeader('X-MC-Tags', $header->getValue());
+                $tags[] = $header->getValue();
                 $headers->remove($name);
             } elseif ($header instanceof MetadataHeader) {
                 $metadata[$header->getKey()] = $header->getValue();
@@ -47,6 +48,10 @@ private function addMandrillHeaders(Message $message): void
             }
         }
 
+        if ($tags) {
+            $headers->addTextHeader('X-MC-Tags', implode(',', $tags));
+        }
+
         if ($metadata) {
             $headers->addTextHeader('X-MC-Metadata', json_encode($metadata));
         }