https://bugs.cacert.org/view.php?id=436

www/index.php

@@ -484,7 +484,13 @@
 			if($checkemail != "OK")
 			{
 				$id = 1;
-				$_SESSION['_config']['errmsg'] .= _("Email Address given was invalid, or a test connection couldn't be made to your server, or the server rejected the email address as invalid")."<br>\n$checkemail<br>\n";
+				if (substr($checkemail, 0, 1) == "4") 
+				{
+					$_SESSION['_config']['errmsg'] .= _("The mail server responsible for your domain indicated a temporary failure. This may be due to anti-SPAM measures, such as \"greylisting\". Please try again in a few minutes.");
+				} else {
+					$_SESSION['_config']['errmsg'] .= _("Email Address given was invalid, or a test connection couldn't be made to your server, or the server rejected the email address as invalid");
+				}
+				$_SESSION['_config']['errmsg'] .= "<br>\n$checkemail<br>\n";
 			}
 		}