Marketpress not sending any email notifications after WP 4.0 upgrade


Marketpress is no longer generating email notifications to either admin or customers. No email message is received by us when an order is placed, and the customer doesn't get an email when they place their order, or when we mark the order as "Shipped". This has always worked just fine until sometime just after 9/1. I think it has to be the Wordpress 4.0 upgrade because the first time the notifications stopped going out was on 9/4, just after the upgrade.

How do we fix this issue?