Is the email you see this in a plain text email?
or does it have the eCart display in it using eCart formatting?
if is plain text, it is a paypal generated email, thus a paypal setting.
if the email has a cart display using eCart formatting, it is an eCart generated email, look in the webassist/email/ folder for a file named waue_confim_php.php to change the from email address.