Emails not showing up

My wife had been corresponding with her cousin. She is using a MacBook Air 2017 model. Recently her cousin has not been receiving my wife's emails. They do not bounce back so I assume they are getting through.

Prior to her current Mac she was using a MacBook Air 2011. Emails from this Mac are getting through. I have sent her email from my iPhone 7 and they have been getting through.

Her cousin is using a PC with Windows.

Any solutions?

via ehMac.ca https://ift.tt/2FUfrbG