sinksmtp: hackily log more detailed information about MX lookup failures

This is a gory hack in several ways that I am not going to discuss right
now, but I've decided that I want this information so I have shoved it in.
sinksmtp: fix bad logging for some bad MX lookups

It's a long story but basically I screwed up and the code was too complex.
Now the code is simpler and there is a big comment so I'll remember what
it's doing.