CAN NOT SEND EMAILS TO YAHOO AND GMAIL ACCOUNTS

For approximately one month one of the members of our company has not been able to send emails to both Yahoo and Gmail. Each time she sends and email this error message comes up:

This message was created automatically by mail delivery software.

A message that you sent could not be delivered to one or more of its recipients. This is a permanent error.

dnlserban@yahoo.com Error, ERROR_CODE :554, ERROR_CODE :5.7.9 Message not accepted for policy reasons. Seehttp://postmaster.yahoo.com/errors/postmaster-28.html

Received:from mail.zoho.com by mx.zohomail.com

with SMTP id 1403539838461900.5810406410839; Mon, 23 Jun 2014 09:10:38 -0700 (PDT)

Message-ID:<146c97e55c6.6482492209605074985.8297917551193659712@yahoo.com>

We just clicked on the advice link but what exactly does " we recommend that you switch to sending mail from your own domain " mean?

How can this error be fixed? As at the moment she can not use her email address at all!

We hope to hear from you soon.

Best,