thanks for the info but there is no as unread marked message in the sent items.
just to answer your question even when the problem is solved now:
There were no filters set
Junk Mail Folder is cleared
No Mail distribution policy
As i write - strange issue but solved at the e…
I know that there is a solution. However desperate users (and admins) called me more than once because of this problem. My described steps helped in most cases.
i made now an new folder and move all mails to the folder as you recommended.
The funny thing was that the system tooked over the unread message into the new folder,
so i started to move the mails in 1000 mails blocks (iv'e got 20000 mails) back to the root folder until
i saw the 1 unread marked mail in the root folder again.
After playing a little bit with moving i found at the end the one mail which made the problem - but the mail was marked as read.
So i mark it to unread (then i've got 2 unread mails) and marked it as read a again and then
i've got NO unread mails….
Very strange but at the end the problem is solved now.
Thanks for support at all of you!