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

Showing Custom Error Message - Web ADI

There are two ways to do this.

1>  Simplest way is to change the 'API Returns' attribute to 'FND Message Code' and use raise_application_errro to raise errors

 Desktop Integration Manager > Manage Integrators
  >> Search the Integrator and go to the 'Interfaces' screen and select the Interface
     >> Change the 'API Returns' to 'FND Message Code' if it is null or 'Error Message'
 
>> Now change the exception section of the related API packaged procedure  

when ex_web_adi1 then
   raise_application_error(-20001,'Web ADI Error Message: '||lc_emp_number);

2> Change the exception section of the related API packaged procedure as below

when ex_web_adi1 then
   lc_err_msg := 'Web ADI Error Message: '||lc_emp_number;
   fnd_message.set_name('XXCMN', lc_err_msg);
   --fnd_message.set_token('ERROR_MESSAGE', lc_err_msg);
   lc_mesg := fnd_message.get;     
   raise_application_error(-20001, lc_mesg);