Folders, when collapsed can hide important emails in child folders
If you have a folder with several child folders where email gets auto-sorted into the child folder, it's extremely easy to miss this new/important emails if the parent folder in the left panel gets collapsed (which happens automatically if you enter outlook's web app through the app launcher) because the parent folder has ZERO indication that any of the folders nested within it have received any emails, while the hidden child folder entries do.
I've missed several important time-sensitive emails as a result of parent folders getting collapsed without being noticed.
Peter heissig commented
unfortunately I missend not only one mail cause i did use Rules and parent folder was not marked as bold ;-(