Sending email apparently from a particular address, as described, is so simple I can't believe two things: 1, that Posterous was set up to let that happen and 2

Sending email apparently from a particular address, as described, is so simple I can't believe two things: 1, that Posterous was set up to let that happen and 2, that in 2010, the email system is still so dumb that I can send mail with any sender address that to the majority of people would be indistinguishable from mail genuinely from the sending address.