Email Relay is only available to Salesforce.com Enterprise and Unlimited Editions.
As of April 2007, Email Relay is not visible by default in Salesforce.com. You must first ask your Salesforce.com account executive to make the "Email Relay" option visible in your Setup area.
Salesforce.com may reply with a series of questions before enabling this feature. The questions (and the desired answers) are here:
Relay for all domains; relay to
In Salesforce.com, go to "Setup > Administration Setup > Email Administration > Email Relay Activation".
If the "Email Relay Activation" option isn't visible, contact your Salesforce.com account executive.
Use "sfdcrelay.ihance.net" as the host, and fill in the rest of the fields like so:
Under Setup, navigate to Email Administration > Deliverability.
In the section Email Security Compliance, uncheck the box next to Enable Sender ID Compliance. If you fail to do this, emails will be delivered, but not tracked. This modifies how emails are sent from Salesforce so they are no longer sent from email@example.com.