How would you like a solution to know if the user received and opened the email ?
I have a solution developed for a marketing module that lets me send an email in AIM and when the recipient receives and opens the email a return connection is made with your AIM server updating your outgoing Email object with the status of "Delivered" or "Opened"
This way you can absolutely guarantee email delivery and implement further workflow rules activating other processes based on the result
If you are interested PM me, unfortunately on this one there there will be a cost as I m trying to recover the development costs.