I liked the premise and enjoyed most of the letter exchanges, but the rest fell flat for me. 

A super predictable plot with lots of holes and things that just didn't make sense, the FMC was unbelievably dense, and some of the writing was not great.

What annoyed me the most was the search for the MMC.

For example, flying clear across the country for one day just to knock on a couple of doors and ask about someone who moved away years ago?

Nonsensical and outrageous.