I recieved an email bounce notification stating, 550 Sorry, your HELO/EHLO greeting must resolve (#5.7.1)
What does this mean?

This shows up due to a flaw in Outlook and Outlook Express, it is caused by the Fully Qualified Domain Name not being populated in the HELO field.  QWK.net servers check for the FQDN as a means to prevent SPAM.

To resolve this issue you can enter an FQDN in windows and the problem will go away.

In Windows XP

Click START

Click Control Panel

Click Network Connections

Right Click your Internet Connection

Click Properties

Click Internet Protocol (TCP/IP) then click Properties

Click Advanced

Click the DNS tab

In the field titled "DNS suffix for this connection:" enter mail.yourdomain.com

Substitute yourdomain with the domain of your email address

You will be able to send email as normal

For Windows 7 follow these instructions

Click the Start/Windows Button

Click Control Panel

Click "View network status and tasks"

Click Change Adapter Settings

Right Click your network connection and click Properties

Click on "Internet Protocol Version 4 (TCP/IPv4)" and click Properties

Click Advanced

Click the DNS tab

In the field titled "DNS suffix for this connection:" enter mail.yourdomain.com

Substitute yourdomain with the domain of your email address

You will be able to send email as normal