http://netbsd.i2p/docs/guide/en/chap-mail.html
Postfix, if correctly configured, will take care of this when it
transfers the messages. You'll probably also want to configure postfix in order to send the e-mails
to the provider's mail server, using it as a relay . In the configuration described in
this chapter, postfix does not directly contact the recipient's
mail server (as previously described) but relays all its mail to
the provider's...