Not Reciving emails from one of the client

Hi, I was getting regular emails from one of my clients in the past now for the last 6 months i am not getting any communocanitons from them it is furstrating becuse they say it's sent but i do no get them  i have changed my email to g mail for the communications but still it is furstrating .... would prefer them to come to comcast.net  need guidance

Thanks