Believe it or not, that question did cross my mind. But then I thought it would be worth asking anyway, for the sake of knowing how to have better control of the headers.
Once I saw a freeware app that stood between the e-mail program and the SMTP, acting as a filter, so you could actually filter, search and replace whatever your e-mail program put in the headers. So I thought: I can make my own headers in PHP, but I cannot hide the recipient's e-mail address like Calypso.
More like a case study, actually.
Luciano ES
Santos, SP - Brasil