Are you working from the All tab and trying to email a draft Purchase order?
The capability to email a draft PO was the result of a functionality having been overlooked. Previous to the navigation change, POs could not be emailed unless they had been completed, which was always the intention. The Manage button was then created, which meant that you could send a PO email, even in draft, which was the aspect that was missed. This has now been corrected.
To complete a PO, please click Manage and select 'Mark as complete'. You will then be able to send the email. It will likely make this area easier to use if you work in the Draft and Complete tabs, instead of the All tab.