By default, the open relay option for the mail server is disabled in Parallels Plesk Panel. You can enable it by using the
root.controls.lock file located in
PRODUCT_DATA_D\var directory. However we do not recommend enabling this option because an open relay can make it possible for an unscrupulous senders to route large volumes of spam.
To enable open relay, follow these steps:
/server/mail.php3:relay_openline and save the file.