Problem
After updating the Metadata for Confluence app, your users may no longer receive Metadata notification mails. And/or if you take a look at your Confluence log files, you will see log entries referring to the MetadataMailNotificationJob and complaining about binary incompatibilities.
Solution
If this is the case, you can easily fix it by restarting Confluence. This will ensure that the correct binaries of the currently installed version of Metadata for Confluence are referenced when Confluence reschedules the job during startup.