Some people are emailing me and told but the email address no longer exists. I am receiving other emails with no problem. Can someone help me?

I know of two people who keep trying to email me and or told the address he does not exist. I have not blocked anyone. I do not have any changed settings. They were able to email me one day and not the next. Some can email me from a different device while others cannot. One person can email me from their laptop and desktop from their phone they are told I don’t exist. Does anyone know what is going on here?