Not Everyone Receiving Email Notices When New Blog Is Created

Support, we are having an issue where not everyone is receiving the email notifications when a new blog post is created. The first person we noticed had a Comcast email, so the thought was that it was something on their end. We have worked with them on a number of whitelisting server IP...but it still doesn't seem to be working. Then just recently we heard from 2 other people who don't use Comcast but still aren't receiving them. Any ideas on how we can help resolve this?