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:
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root.controls.lock file./server/mail.php3:relay_open line and save the file.