I don't think you can forge the sender using mail. To do that, consider the -f option of sendmail. Keep in mind that ...
Quote:
Sendmail is not intended as a user interface routine; other programs
provide user-friendly front ends; sendmail is used only to deliver pre-
formatted messages.
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From the man page, sendmail(8).
(edit: fixed typo)
If you want to go really low-level, you can also talk directly SMTP (port 25) to the recipient's MX agent.