То, что вы ищете, это -f
:
sendmail -F "Your name" -f "your-address@example.com" < your-message.txt
со страницы руководства sendmail :
-Ffullname Set the full name of the sender.
-fname Sets the name of the ``from'' person (i.e., the sender of the
mail). -f can only be used by ``trusted'' users (normally
root, daemon, and network) or if the person you are trying to
become is the same as the person you are.