Comply with RFC 5321 and try to lookup A rr, if MX lookup fails

According to RFC 5321, if a host has no MX defined, try A resource record as a last resort. See http://en.wikipedia.org/wiki/MX_record#History_of_fallback_to_A . This patch tries to implement that.
I see that there was already an A lookup made before in Line 160. So now we would have it twice?