If you have a workflow group that has a member with a faulty email address, it could prevent ANY member of the workflow group to receive emails.
You will see something like this in the logs:
2015-06-04 12:37:13,326 WARN org.dspace.workflow.WorkflowManager @ XXXX@XXXX:session_id=3F7C1704BE8F2DB31447FCC8EFF3DD17:notifyGroupofTask:cannot email user group_id=6 workflow_item_id=14\colon; Invalid Addresses
How to solve:
The type of messaging exceptions that appears here should really be in the for loop which iterates over the group members, and not after it:
Also, the stacktrace of the exception should be logged.
This only applies to the STANDARD DSpace workflow and not with the XML Workflow/Configurable workflow.