message-sendmail-envelope-from

message-sendmail-envelope-from is a variable defined in `message.el'.
Its value is

nil

Documentation:
*Envelope-from when sending mail with sendmail.
If this is nil, use `user-mail-address'. If it is the symbol
`header', use the From: header of the message.

You can customize this variable.

This variable was introduced, or its default value was changed, in version 23.2 of Emacs.