Hi,
You might need to get yourself familiar with the extended notification custom handler(s). If you check SWNCONFIG and then open the category, there you will find place where you can change the handlers for custom handlers. => Coding required. I am not sure if there is an easier way.
Anyhow, I have faced this same requirement. Eventually I recommended that if the users don't like certain emails they can block the sender or set up some kind of filter in their email client (in that case it was Outlook). I think that we even had this instruction text in the notification email itself ("If you don't want to receive this email, blah blah...".
Regards,
Karri