Symptoms

Issue was found when attempting to start a new workflow. It was discovered that the workflow for a particular list would work as expected, but no notification emails were being sent to the corresponding users.
 

Diagnoses

This issue can be caused when a user still has group rights applied in K2, but is no longer existent due to lack of use / removal of rights. This was confirmed as there was a user who was no longer needed in the environment.
 

Resolution

Issue was resolved by removing the user from SharePoint and sync groups was run. Email notifications were again enabled and the issue did not occur after.