Collapse search bar by default like other Office apps
Like the other Office apps Excel, Word, and PowerPoint, they have an option where you can collapse the search bar on the window by default. Apparently Outlook doesn't.
Please Microsoft, if you are listening user voice, collapse search bar like other office apps eg. word
Sean Ellis commented
Even better, move it off the title bar completely. The title bar has a set of defined roles and behavior in both Windows and Mac OS which are broken by the inclusion of the search widget.