We came across an Issue that notification remains sitting in INBOX not processed to either PROCESS or DISCARD folder.
Also we noticed the following error after changing the log level to statement.
1. Stop the workflow mailer.
2. Make sure that "Processor Close on Read Timeout" and "Expunge Inbox on Close" parameter is checked (In OAM --> Navigate to Workflow Manager --> Notification Mailers --> Edit --> Advanced --> Step 2 and --> Step 3).
3. Restart the workflow mailer.
4. Retest the Issue.