Email recipient not accepted by the server

I receive the error message every time I try to send an email to my wife and my daughter @ I am able to send emails to others  that use,,, with no problems. For my wife and daughter I just copy the message and logged on to my live account and they were sent with no problems.  


What mail service provider are you using yourself?

I have the same problem - and using Google - Gsuite for Business
Any solution?