Error in sending emails

I setup mailjet smtp serer deatils in mautic and success message is shown but when try to send any email message to any emails …
this error message is showing…
Failed to send to 0121anujsharma@gmail.com: Expected response code 354 but got code “554”, with message "554 5.5.1 Error: no valid recipients "
what to do or how to resolve this error?