diff --git a/Header/Headers.php b/Header/Headers.php
index 1a7b4b2..201a7a5 100644
--- a/Header/Headers.php
+++ b/Header/Headers.php
@@ -141,8 +141,8 @@ public function add(HeaderInterface $header): self
             '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/Tests/Header/HeadersTest.php b/Tests/Header/HeadersTest.php
index 89d8be0..5da2729 100644
--- a/Tests/Header/HeadersTest.php
+++ b/Tests/Header/HeadersTest.php
@@ -243,4 +243,18 @@ public function testToArray()
             "Foo: =?utf-8?Q?aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa?=\r\n =?utf-8?Q?aaaa?=",
         ], $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());
+    }
 }