mail-specify-envelope-from

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

nil

Documentation:
If non-nil, specify the envelope-from address when sending mail.
The value used to specify it is whatever is found in
the variable `mail-envelope-from', with `user-mail-address' as fallback.

On most systems, specifying the envelope-from address is a
privileged operation. This variable affects sendmail and
smtpmail -- if you use feedmail to send mail, see instead the
variable `feedmail-deduce-envelope-from'.

You can customize this variable.

This variable was introduced, or its default value was changed, in version 21.1 of Emacs.