How should I respond to being addressed as "Dr. [MyName]" when I don't have a PhD?
This is a very common experience, and mostly I think that you should ignore it.  From my experience, the primary reasons that you will get addressed as Dr. (or Prof.) are:


The sender doesn't know who you are and doesn't really care ("Honorable Dr. please send your excellent paper to our predatory journal sir!")
They are using a form or other automated system that is automatically inserting the title. 
Somebody who does care but isn't sure of your status "rounds you up," figuring they are less likely to offend by too much honor than by too little.


If the person is somebody you actually want to have a relationship with or are concerned about misleading, then you can correct them, but for the most part just don't worry about it.