user@ydomain.com Invalid Address, ERROR_CODE :550, ERROR_CODE :Please turn on SMTP Authentication in your mail client.

user@ydomain.com Invalid Address, ERROR_CODE :550, ERROR_CODE :Please turn on SMTP Authentication in your mail client.

i got this mail when i mail to user@mydomain.com

user@mydomain.com Invalid Address, ERROR_CODE :550, ERROR_CODE :Please turn on SMTP Authentication in your mail client.
sender1.zohomail.com [XX.XX.XX.XX]:46616 is not permitted to relay through this server without authentication.



i don't know about this.
please anyone help me