Variable: mail-specify-envelope-from

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'.