Intermittent IMAP/S error: This operation is not allowed on a closed folder

SYMPTOM

A Mule Application intermittently shows exceptions similar to the trace shown below.

java.lang.IllegalStateException: This operation is not allowed on a closed folder
    at com.sun.mail.imap.IMAPFolder.checkOpened(IMAPFolder.java:409)
    at com.sun.mail.imap.IMAPFolder.getMessage(IMAPFolder.java:1376)
    at javax.mail.Folder.getMessages(Folder.java:880)

CAUSE

A common cause of this kind of issues is that the folder is being modified concurrently using the same account and credentials thus leaving the operations in an inconsistent state. 

SOLUTION

Verify that the account and credentials being used to process e-mails in your Mule Application is not shared with any other application, process or person.