diff --git a/htdocs/core/class/CMailFile.class.php b/htdocs/core/class/CMailFile.class.php
index 91e7d127486..ac4f4c0da39 100644
--- a/htdocs/core/class/CMailFile.class.php
+++ b/htdocs/core/class/CMailFile.class.php
@@ -81,19 +81,20 @@ class CMailFile
 	 */
 	public $errors = array();
 
-	public $smtps; // Contains SMTPs object (if this method is used)
-	public $phpmailer; // Contains PHPMailer object (if this method is used)
+
+	/**
+	 * @var SMTPS (if this method is used)
+	 */
+	public $smtps;
+	/**
+	 * @var Swift_Mailer (if the method is used)
+	 */
+	public $mailer;
 
 	/**
 	 * @var Swift_SmtpTransport
 	 */
 	public $transport;
-
-	/**
-	 * @var Swift_Mailer
-	 */
-	public $mailer;
-
 	/**
 	 * @var Swift_Plugins_Loggers_ArrayLogger
 	 */
@@ -108,9 +109,13 @@ class CMailFile
 	//! Defined background directly in body tag
 	public $bodyCSS;
 
+	/**
+	 * @var string	Message-ID of the email to send (generated)
+	 */
 	public $msgid;
 	public $headers;
 	public $message;
+
 	/**
 	 * @var array fullfilenames list (full path of filename on file system)
 	 */
@@ -579,12 +584,15 @@ class CMailFile
 			//$this->message = new Swift_SignedMessage();
 			// Adding a trackid header to a message
 			$headers = $this->message->getHeaders();
+
 			$headers->addTextHeader('X-Dolibarr-TRACKID', $this->trackid.'@'.$host);
 			$this->msgid = time().'.swiftmailer-dolibarr-'.$this->trackid.'@'.$host;
 			$headerID = $this->msgid;
 			$msgid = $headers->get('Message-ID');
 			$msgid->setId($headerID);
-			$headers->addIdHeader('References', $headerID);
+
+			// Add 'References:' header
+			//$headers->addIdHeader('References', $headerID);
 
 			if (!empty($moreinheader)) {
 				$moreinheaderarray = preg_split('/[\r\n]+/', $moreinheader);
@@ -727,7 +735,6 @@ class CMailFile
 		}
 	}
 
-
 	/**
 	 * Send mail that was prepared by constructor.
 	 *
@@ -1536,7 +1543,7 @@ class CMailFile
 			// References is kept in response and Message-ID is returned into In-Reply-To:
 			$this->msgid = time().'.phpmail-dolibarr-'.$trackid.'@'.$host;
 			$out .= 'Message-ID: <'.$this->msgid.">".$this->eol2; // Uppercase seems replaced by phpmail
-			$out .= 'References: <'.$this->msgid.">".$this->eol2;
+			//$out .= 'References: <'.$this->msgid.">".$this->eol2;
 			$out .= 'X-Dolibarr-TRACKID: '.$trackid.'@'.$host.$this->eol2;
 		} else {
 			$this->msgid = time().'.phpmail@'.$host;
diff --git a/htdocs/core/class/smtps.class.php b/htdocs/core/class/smtps.class.php
index 150414b3c4d..572bc38f363 100644
--- a/htdocs/core/class/smtps.class.php
+++ b/htdocs/core/class/smtps.class.php
@@ -651,12 +651,9 @@ class SMTPs
 	 */
 	public function sendMsg()
 	{
-		global $conf;
-
 		require_once DOL_DOCUMENT_ROOT.'/core/lib/geturl.lib.php';
-		/**
-		 * Default return value
-		 */
+
+		// Default return value
 		$_retVal = false;
 
 		// Connect to Server
@@ -1451,10 +1448,10 @@ class SMTPs
 
 		$trackid = $this->getTrackId();
 		if ($trackid) {
-			// References is kept in response and Message-ID is returned into In-Reply-To:
 			$_header .= 'Message-ID: <'.time().'.SMTPs-dolibarr-'.$trackid.'@'.$host.">\r\n";
-			$_header .= 'References: <'.time().'.SMTPs-dolibarr-'.$trackid.'@'.$host.">\r\n";
 			$_header .= 'X-Dolibarr-TRACKID: '.$trackid.'@'.$host."\r\n";
+			// References and In-Reply-To: will be set by caller
+			//$_header .= 'References: <'.time().'.SMTPs-dolibarr-'.$trackid.'@'.$host.">\r\n";
 		} else {
 			$_header .= 'Message-ID: <'.time().'.SMTPs@'.$host.">\r\n";
 		}
@@ -1465,10 +1462,6 @@ class SMTPs
 			$_header .= $this->getMoreInHeader(); // Value must include the "\r\n";
 		}
 
-		//$_header .=
-		//                 'Read-Receipt-To: '   . $this->getFrom( 'org' ) . "\r\n"
-		//                 'Return-Receipt-To: ' . $this->getFrom( 'org' ) . "\r\n";
-
 		if ($this->getSensitivity()) {
 			$_header .= 'Sensitivity: '.$this->getSensitivity()."\r\n";
 		}
@@ -1493,6 +1486,7 @@ class SMTPs
 		$_header .= 'X-Dolibarr-Option: '.($conf->global->MAIN_MAIL_USE_MULTI_PART ? 'MAIN_MAIL_USE_MULTI_PART' : 'No MAIN_MAIL_USE_MULTI_PART')."\r\n";
 		$_header .= 'Mime-Version: 1.0'."\r\n";
 
+		// TODO Add also $this->references and In-Reply-To
 
 		return $_header;
 	}
diff --git a/htdocs/ticket/class/ticket.class.php b/htdocs/ticket/class/ticket.class.php
index cdfb8beef07..3d231680b21 100644
--- a/htdocs/ticket/class/ticket.class.php
+++ b/htdocs/ticket/class/ticket.class.php
@@ -3008,10 +3008,24 @@ class Ticket extends CommonObject
 
 				$moreinheader = 'X-Dolibarr-Info: sendTicketMessageByEmail'."\r\n";
 				if (!empty($this->email_msgid)) {
-					$moreinheader .= 'References: <'.$this->email_msgid.'>'."\r\n";
+					// We must also add 1 entry In-Reply-To: <$this->email_msgid> with Message-ID we respond from (See RFC5322).
+					$moreinheader .= 'In-Reply-To: <'.$this->email_msgid.'>'."\r\n";
+				}
+
+				// We should add here also a header 'References:'
+				// According to RFC5322, we should add here all the References fields of the initial message concatenated with
+				// the Message-ID of the message we respond from (but each ID must be once).
+				$references = '';
+				// @TODO
+				// Retrieve source References to do  				$references .= (empty($references) ? '' : ' ').Source References
+				// If No References is set, use the In-Reply-To for $references .= (empty($references) ? '' : ' ').Source In-reply-To
+				$references .= (empty($references) ? '' : ' ').'<'.$this->email_msgid.'>';
+				if ($references) {
+					$moreinheader .= 'References: '.$references."\r\n";
 				}
 
 				$mailfile = new CMailFile($subject, $receiver, $from, $message, $filepath, $mimetype, $filename, $sendtocc, '', $deliveryreceipt, -1, '', '', $trackid, $moreinheader, 'ticket', '', $upload_dir_tmp);
+
 				if ($mailfile->error) {
 					setEventMessages($mailfile->error, null, 'errors');
 				} else {