Not receiving emails

Hi there, I have configured email for one of my client. I am able to sent email from Webmail but not able to receive email. I get the following error:- 550: Please turn on SMTP Authentication in your mail client. sonic314-19.consmr.mail.sg3.yahoo.com [106.10.240.143]:34876 is not permitted to relay through this server without authentication. Can you please suggest what the issue is and what is the remedy? I am using Webmail for composing/receiving emails. Thanks in Advance. Regards Baroda