Try this extension for Google Chrome - it removes the space on the right:
Webmail Ad Blocker

Webmail Ad Blocker - Chrome Web Store

What I did was to remove the uBlock Origin extension, then install the Webmail Ad Blocker.
Refresh Outlook.com and check that the adds had gone and the space on the right was ‘reclaimed’ (it works).
I then reinstalled uBlock Origin (optional) as I rate this blocker highly.



Re EM Client – my main email client is Outlook 2016 and have EM Client as a back up.
In my opinion, it is one of the better free email clients available – give it a try.