Bug 323739

MDN's as of now go to inbox by default.

This is not ideal as they are not close to the emails themselves which are in the sent items folder making it harder to correlate emails with MDN's. This makes it hard to keep track of who has opened the email and who has not when sending more than one email with a similar subject.

A way around this is to group MDN's with the email in the sent items(like a thread) and have an icon like the icon in the status to display the status of the email(just like we have one to notify us if a message is read or has an attachment).

The actual MDN should be viewable from the email so the workflow for viewing a MDN would be:
- Open sent email
- Click view MDNs
- Window opens with a list of MDNs for that email.

However the status of the email can be seen by a 'read by recipient' or 'delivered and unread' or 'deleted before opening' icon in the status of the message. 

Reproducible: Always