Make it possible to add return path for mails (#95)

core/email.class.inc.php

@@ -304,6 +304,10 @@ class EMail
 			$oHeaders = $this->m_oMessage->getHeaders();
 			switch(strtolower($sKey))
 			{
+				case 'return-path':
+					$this->m_oMessage->setReturnPath($sValue)
+					break;
+
 				default:
 				$oHeaders->addTextHeader($sKey, $sValue);
 			}