The 'HasOptedOutOfEmail' fields on the Salesforce Lead and Contact objects must be mapped to the portal Contact.EmailOptOut field in order for emails to be CAN-SPAM compliant.

In some Salesforce installations, this field is not visible by default. To enable your portal user integration account to access this field in Salesforce, go to Setup->Administration Setup->Manage Users->Profiles then select the profile of your portal integration user account.

Then under "Field Level Security", click on the Lead and Contact "View" links and ensure that the 'HasOptedOutOfEmail' field is visible to this profile.