OpenIDM includes Activiti 5.15. If a mailtask can't be completed in a workflow, an exception will be thrown, aborting the workflow. There is no way to handle this inside an activit workflow.
For example, sending an email to a manager "please approve" will render the processinstance unusable if the mailserver is unreachable, one of several recipients doesn't exist any longer, etc.
Activiti 5.16 has the option to ignore mail task issues or handle them inside the workflow (see "ignoreException" and "mailException").
OpenIDM trunk compiles without issues using activiti 5.16, so I don't expect this to be a major issue.