Every time I have a new account using Outlook, I always forget to add it back, since it's not done automagically anymore. Once I remember I want it back, I have to look up how to do it!
Sad thing is it's pretty easy.
At the bottom of the list of folders in your account, there's a Search Folder.
Right click Search Folders, and click on New Search Folder.
Make sure Unread mail is selected in the Search Folder box and click OK.
The folder will appear on Search Folders, and then you can right click it and select Show in Favorites. Now you can find the email you left unread so you can deal with it later.... about a week ago. :)
