diff --git a/program/lib/Roundcube/rcube_mime.php b/program/lib/Roundcube/rcube_mime.php
index e1965df29d..22e4aa63dd 100644
--- a/program/lib/Roundcube/rcube_mime.php
+++ b/program/lib/Roundcube/rcube_mime.php
@@ -162,6 +162,11 @@ public static function decode_mime_string($input, $fallback = null)
     {
         $default_charset = $fallback ?: self::get_charset();
 
+        // RFC 6532: detect raw UTF-8 in headers to avoid wrong charset conversion
+        if ($fallback !== false && mb_check_encoding($input, 'UTF-8') && preg_match('/[\x80-\xFF]/', $input)) {
+            $default_charset = 'UTF-8';
+        }
+
         // rfc: all line breaks or other characters not found
         // in the Base64 Alphabet must be ignored by decoding software
         // delete all blanks between MIME-lines, differently we can
diff --git a/tests/Framework/MimeTest.php b/tests/Framework/MimeTest.php
index 1cf3c63713..348f8f7cf2 100644
--- a/tests/Framework/MimeTest.php
+++ b/tests/Framework/MimeTest.php
@@ -216,6 +216,23 @@ public function test_header_decode_qp()
         }
     }
 
+    /**
+     * Test decoding of raw UTF-8 headers (RFC 6532)
+     * Uses rcube_mime::decode_mime_string()
+     */
+    public function test_header_decode_raw_utf8()
+    {
+        // Raw UTF-8 Korean with wrong fallback charset - should be preserved
+        $korean = "\xEC\x95\x88\xEB\x85\x95\xED\x95\x98\xEC\x84\xB8\xEC\x9A\x94"; // 안녕하세요
+        $res = \rcube_mime::decode_mime_string($korean, 'ISO-8859-1');
+        $this->assertSame($korean, $res, 'Raw UTF-8 header with Latin-1 fallback');
+
+        // Latin-1 high bytes (not valid UTF-8) - should still be converted
+        $latin1 = "caf\xE9"; // café in ISO-8859-1
+        $res = \rcube_mime::decode_mime_string($latin1, 'ISO-8859-1');
+        $this->assertSame('café', $res, 'Latin-1 header should be converted');
+    }
+
     /**
      * Test headers parsing
      */