Farid,
Once PlanetPress Workflow has sent the email on its way, it's completely out of our hands what actually happens to it. Email delivery failures will come back to whatever email address was configured as the "From" address (POP3) or the Outlook Email Address (Outlook connection). PlanetPress itself has no log or notification that this happens.
You could automate this by creating a process with an Email Input, that would read the delivery failure notifications... However, this can be difficult as the notifications are not always in the same form, and they don't always have information beyond the email address it was sent to. Also, you would have to watch out for people actually replying to the email!