Hi Hugo,
I don't believe this is possible with the Email Settings API. There are several restrictions:
Note: A valid value of
forwardTo is an email
address that has been already verified to be used as a forwarding
address in the Google Mail account, or one that fulfills one of the
following criteria:
1. It belongs to the same domain,
2. It belongs to a subdomain of the same domain, or
3. It belongs to a domain alias configured as part of the same Google Apps account.
If the value of
forwardTo doesn't adhere to one of these rules, setting up forwarding via the API will fail.
Please refer to
http://code.google.com/googleapps/domain/email_settings/developers_guide_protocol.html#GA_email_forwarding_main for more information.
Regards,
Brian
Brian McNamara, Certified Google Apps Deployment Specialist