+JH/19 Two additional event types: msg:rcpt:defer and msg:rcpt:host:defer. Both
+ in transport context, after the attempt, and per-recipient. The latter type
+ is per host attempted. The event data is the error message, and the errno
+ information encodes the lookup type (A vs. MX) used for the (first) host,
+ and the trailing two digits of the smtp 4xx reponse.
+