To view header info in Office 2016 from the email list, right click and click ‘View Source’ and this will allow you to see all the email info.
To view header info from the Web Client click in to the email and the drop down beside the ‘Reply All’ button in the top right there’s a ‘View Message Detail’
While helping our email admin troubleshoot why SFDC emails where hitting the Spam Folder I noticed that there wasn’t accurate info on the internet as to how to view this in the new clients.