Question: Why do some yahoo.com email users not receive emails I send them that are "mass mail"? Some do, some don't-why 

Answer: I would guess your mail might get filtered as spam. Some spam filters have rules about how many people can be on a particular message as recipients. Therefore if you have not been safe listed by a user (added to the address book) you will be filtered if your send to field has more than 7 addresses in it.
This question from the Yahoo Q&A forum is categorized in the topic: Sports