diff --git a/src/Symfony/Component/Mime/Header/Headers.php b/src/Symfony/Component/Mime/Header/Headers.php
index 57d730127ba0..48c73e0ccf82 100644
--- a/src/Symfony/Component/Mime/Header/Headers.php
+++ b/src/Symfony/Component/Mime/Header/Headers.php
@@ -147,6 +147,8 @@ public function addHeader(string $name, mixed $argument, array $more = []): stat
             $method = 'addTextHeader';
         } elseif ('addIdentificationHeader' === $method) {
             $method = 'addIdHeader';
+        } elseif ('addMailboxListHeader' === $method && !\is_array($argument)) {
+            $argument = [$argument];
         }
 
         return $this->$method($name, $argument, $more);
diff --git a/src/Symfony/Component/Mime/Tests/Header/HeadersTest.php b/src/Symfony/Component/Mime/Tests/Header/HeadersTest.php
index 2381f28818f6..b1892978c1a0 100644
--- a/src/Symfony/Component/Mime/Tests/Header/HeadersTest.php
+++ b/src/Symfony/Component/Mime/Tests/Header/HeadersTest.php
@@ -69,6 +69,7 @@ public function testAddHeader()
     {
         $headers = new Headers();
         $headers->addHeader('from', ['from@example.com']);
+        $headers->addHeader('reply-to', 'reply@example.com');
         $headers->addHeader('return-path', 'return@example.com');
         $headers->addHeader('foo', 'bar');
         $headers->addHeader('date', $now = new \DateTimeImmutable());
@@ -77,6 +78,9 @@ public function testAddHeader()
         $this->assertInstanceOf(MailboxListHeader::class, $headers->get('from'));
         $this->assertEquals([new Address('from@example.com')], $headers->get('from')->getBody());
 
+        $this->assertInstanceOf(MailboxListHeader::class, $headers->get('reply-to'));
+        $this->assertEquals([new Address('reply@example.com')], $headers->get('reply-to')->getBody());
+
         $this->assertInstanceOf(PathHeader::class, $headers->get('return-path'));
         $this->assertEquals(new Address('return@example.com'), $headers->get('return-path')->getBody());