Unable to send an email get rejected by the server

Unable to send an email get rejected by the server

Hello everyone, 

thank you kindly for reading my question/problem today. I can successfully send emails from my IPad however I cannot send emails from my IPhone, it says "A copy has been placed in your outbox. The sender address " rhys@rhyssmart.com" was rejected by the server". 

How can I solve this issue?

Thank you.