Are you sure there are bounce reports (failed Delivery Status Notifications) in your mailbox?
Usually such emails have a subject similar to "Delivery Status Notification (Failed)".
If there are, and Mail.dll's Bounce
class fails to recognize them, please send those to us for examination.
Easiest way would be to simply send an email, with such email attached (zipped) to support address (you can find it in the page footer below).
You can find details how to do that here:
https://www.limilabs.com/blog/i-have-problems-parsing-the-message
P.S.
Your code performs server search twice.
There is no need for this check:
if (imap.Search(Flag.UnSeen).Count > 0)