I just took a quick look and I have both "wt.mail.from" and "wt.notify.notificationSenderEmail" set equal to the Windchill administrator's email address.
From the documentation:
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Setting Sender E-mail Address
For e-mail messages to be sent, some mail servers require that a valid e-mail address be specified in the From header of the e-mail message. Windchill uses the value specified in the wt.mail.from property in the wt.properties file as the default sender of e-mail messages. Similarly, the wt.notify.notificationSenderEmail property is used to specify the e-mail address used as the sender of all event-based notifications.
By default, the wt.mail.from property is set to Postmaster@<hostname>, where <hostname> is the value specified in the java.rmi.server.hostname property. The wt.notify.notificationSenderEmail property defaults to the value of the wt.admin.defaultAdministratorName property.
PTC suggests that the wt.mail.from and wt.notify.notificationSenderEmail properties both be set to the e-mail address of the Windchill administrator or some other authorized user.