FIX Introduce hidden option MAIL_PREFIX_FOR_EMAIL_ID to solve pb of

tracking email.

htdocs/core/lib/functions.lib.php

@@ -290,8 +290,15 @@ function GETPOST($paramname,$check='',$method=0,$filter=NULL,$options=NULL)
  */
 function dol_getprefix()
 {
+    global $conf;
+    
 	if (isset($_SERVER["SERVER_NAME"]) && isset($_SERVER["DOCUMENT_ROOT"]))
 	{
+	    if (! empty($conf->gobal->MAIL_PREFIX_FOR_EMAIL_ID))
+	    {
+	        if ($conf->gobal->MAIL_PREFIX_FOR_EMAIL_ID == 'SERVER_NAME') return $_SERVER["SERVER_NAME"];
+	        return $conf->gobal->MAIL_PREFIX_FOR_EMAIL_ID;
+	    }
 		return dol_hash($_SERVER["SERVER_NAME"].$_SERVER["DOCUMENT_ROOT"].DOL_DOCUMENT_ROOT.DOL_URL_ROOT);
 		// Use this for a "clear" cookie name
 		//return dol_sanitizeFileName($_SERVER["SERVER_NAME"].$_SERVER["DOCUMENT_ROOT"].DOL_DOCUMENT_ROOT.DOL_URL_ROOT);