The postmaster is the server that your e-mail is delivered to to get to the recipient
if the e-mail address is not correct in every detail, that server ( postmaster ) will deny it and send back that message
the address has to be exact in case , underscore, punctuation, spacing or no spacing etc
even a comma before the address will get that response
one way is to find an e-mail from the address you are sending to , and copy/paste it in the address line or if you are using a contact me -email address copy/paste it in the receiver line
if you open up the message from the postmaster , it will explain possible reason s for non delivery