[fullfactor]

Originally Posted by ETDC

Sorry, but I think that's rubbish - the user will be inviting a friend by entering an email address that they've probably already written to. How could their own email be marked as spam over one from an unknown, new source?...

To expand on Kaelon's reply: Email is easy to forge. DNS is not. Anyone can send an email with your address in the From header, so many modern spam detection systems place a greater importance on authenticating the originating server. In simple terms, mechanisms like SPF and Reverse DNS, when used with an appropriate From address, are used to tell spam detection systems that the server from which the message is being sent is genuine, and that the address in the From header really is the address of the sender.

More information about SPF:
http://www.openspf.org/
http://en.wikipedia.org/wiki/Sender_Policy_Framework

More information about Reverse DNS:
http://en.wikipedia.org/wiki/Reverse_DNS_lookup

You do not need to operate your own DNS server to implement SPF and Reverse DNS. However, you do need to be able to make changes to your domain's DNS record, and your ISP or hosting provider needs to allow and support this.

HTH.