I am able to receive emails, but Windows Live Mail will not let me send emails. I keep getting this error report:

The message could not be sent because the server rejected the sender's email address. The sender's email address was '*** Email address is removed for privacy ***'.

Subject 'Re: New client inquiry.'
Server Error: 550
Server Response: 550 Access denied - Invalid HELO name (See RFC2821 4.1.1.1)
Server: 'mail.thedendoggiedaycare.com'
Windows Live Mail Error ID: 0x800CCC78
Protocol: SMTP
Port: 587
Secure(SSL): No

What do I need to do so that I am able to send emails again?