When opening an e-mail in a new window, outlook opens mailbox again instead.
Vincent Gustin commented
When answering a mail,
When generating a new mail,
it opens in the same interface ...
Before, it opened in a new window (not a tab) ...
This way was much more convenient and fast to use the CTRL + TAB to switch from OutLook (or other) to read / copy / navigate and write / paste in the reply or the new mail.