diff --git a/src/Symfony/Component/Mime/Header/Headers.php b/src/Symfony/Component/Mime/Header/Headers.php
index dba65e0ae101..8db912520a85 100644
--- a/src/Symfony/Component/Mime/Header/Headers.php
+++ b/src/Symfony/Component/Mime/Header/Headers.php
@@ -34,8 +34,8 @@ final class Headers
         'cc' => MailboxListHeader::class,
         'bcc' => MailboxListHeader::class,
         'message-id' => IdentificationHeader::class,
-        'in-reply-to' => IdentificationHeader::class,
-        'references' => IdentificationHeader::class,
+        'in-reply-to' => UnstructuredHeader::class, // `In-Reply-To` and `References` are less strict than RFC 2822 (3.6.4) to allow users entering the original email's ...
+        'references' => UnstructuredHeader::class, // ... `Message-ID`, even if that is no valid `msg-id`
         'return-path' => PathHeader::class,
     ];
 
diff --git a/src/Symfony/Component/Mime/Tests/Header/HeadersTest.php b/src/Symfony/Component/Mime/Tests/Header/HeadersTest.php
index 37e3192fbe94..78c4df8533c9 100644
--- a/src/Symfony/Component/Mime/Tests/Header/HeadersTest.php
+++ b/src/Symfony/Component/Mime/Tests/Header/HeadersTest.php
@@ -280,6 +280,20 @@ public function testToArray()
         ], $headers->toArray());
     }
 
+    public function testInReplyToAcceptsNonIdentifierValues()
+    {
+        $headers = new Headers();
+        $headers->addHeader('In-Reply-To', 'foobar');
+        $this->assertEquals('foobar', $headers->get('In-Reply-To')->getBody());
+    }
+
+    public function testReferencesAcceptsNonIdentifierValues()
+    {
+        $headers = new Headers();
+        $headers->addHeader('References' , 'foobar');
+        $this->assertEquals('foobar', $headers->get('References')->getBody());
+    }
+
     public function testHeaderBody()
     {
         $headers = new Headers();