This is more of a curiosity than anything else:
I did this exercise:
One user downloads all his email from mail.msu.edu to Thunderbird, but always leaves the email at mail.msu.edu without deleting them.
Now he lost some email from his Thunderbird and he wants to download it again.
He goes to mail.msu.edu and marks those emails as "Unread", or "Unseen".
Goes back to Thunderbird and downloads his email.
Thunderbird does not see the emails marked as unread as new email.

So....the unread flag that you set does not really marks the email as unread. It marks it as something else. Am I missing something here?



--
Oscar Castaņeda
Global Observatory for Ecosystem Services
Michigan State University