Well, the "return path" is not a header field. It's more officially called the "envelope from", or "From_". And -f does in fact set it, if you're permitted by the configuration.

Of course, with SMTP as your first hop rather than sendmail, to some degree you have much more flexibility, including setting envelope from/to independent of the header from/to and sender.

-- Randal L. Schwartz, Perl hacker