smtp;550 Access denied - Invalid HELO name (See RFC2821 4.1.1.1)

Some SMTP servers require you to send a valid host name with the HELO command that also is the same as the reverse DNS of the IP address the request originates from.

You can configure the host name the IIS SMTP Server uses through the old IIS6 Management Console. To do so, select the SMTP Server, right click and select Properties, go to the tab Delivery and then click on the button Advanced.

The reverse DNS for your IP address is usually controlled by your ISP.