Notification Mailer Keeps Sending the same closed Notification

The probable reason is the incorrect receipient's incorrect email address.

The SMTP server does not handle invalid email address it should be either null of valid email address

Solution

Go to System Administrator > Workflow > Oracle Applications Manager > Workflow Manager

Click on the 'Notification Mailers' > Workflow Notification Mailer

Edit > Advanced >
    At step 6 create a new TAG with 'Pattern' name as the 'Subject of the Notification' with Action value of 'Ignore'

Ex: Undeliverable             Ignore