diff --git a/mail.php b/mail.php
index 151fb36..52a0bad 100644
--- a/mail.php
+++ b/mail.php
@@ -1119,9 +1119,31 @@ class mail
         throw new \Exception ("Wanted section not found in addHeader", 404);
       $sectionMainID = $sectionID;
     }
-    $this->sections[$sectionMainID]["_headersArray"][][$header] = $value;
-    // TODO : Encode ? Strip ?
-    $this->sections[$sectionMainID]["_headersEML"] .= "$header: $value";
+    if ($header === "Return-Path")
+    {
+      // Must be placed on the first line and must be unique
+      reset ($this->sections[$sectionMainID]["_headersArray"]);
+      if (key_exists (0, $this->sections[$sectionMainID]["_headersArray"]) &&
+          key_exists ("Return-Path", $this->sections[$sectionMainID]["_headersArray"][0]))
+      {
+        // Remove the old one
+        array_shift ($this->sections[$sectionMainID]["_headersArray"]);
+        $this->sections[$sectionMainID]["_headersEML"] =
+          substr ($this->sections[$sectionMainID]["_headersEML"],
+            strpos ($this->sections[$sectionMainID]["_headersEML"], "\n") +1);
+      }
+      array_unshift ($this->sections[$sectionMainID]["_headersArray"],
+        array ($header => $value));
+      $this->sections[$sectionMainID]["_headersEML"] = "$header: $value".
+        $this->sections[$sectionMainID]["_headersEML"];
+    }
+    else
+    {
+      // The "normal" headers are places sequentially at the end of the headers
+      $this->sections[$sectionMainID]["_headersArray"][][$header] = $value;
+      // TODO : Encode ? Strip ?
+      $this->sections[$sectionMainID]["_headersEML"] .= "$header: $value";
+    }
     $this->createMailEML ();
   }