I'm using MDB of type org.jboss.resource.adapter.mail.inflow.MailListener
it was working incorrectly. Sometimes, out of blue, it stopped fetching mail with exception (which I could see after fix from JBAS-3769):

2007-02-02 11:51:52,220 ERROR [MailActivation] Failed to execute folder check, spec=MailActivationSpec(mailServer=10.0.0.1, storeProtocol=imap, mailFolder=INBOX, pollingInterval=60000, messageSelector=null, userName=***, maxMessages=1)
javax.mail.NoSuchProviderException: Invalid protocol: null
at javax.mail.Session.getProvider(Session.java:412)
at javax.mail.Session.getStore(Session.java:511)
at javax.mail.Session.getStore(Session.java:491)
at javax.mail.Session.getStore(Session.java:477)
at org.jboss.resource.adapter.mail.inflow.MailFolder.open(MailFolder.java:69)
at org.jboss.resource.adapter.mail.inflow.MailActivation.run(MailActivation.java:121)
at org.jboss.resource.work.WorkWrapper.execute(WorkWrapper.java:204)
at org.jboss.util.threadpool.BasicTaskWrapper.run(BasicTaskWrapper.java:275)
at EDU.oswego.cs.dl.util.concurrent.PooledExecutor$Worker.run(PooledExecutor.java:743)
at java.lang.Thread.run(Thread.java:595)

problem was fixed, when I applyed this patch:
Index: src/main/org/jboss/resource/adapter/mail/inflow/MailFolder.java
===================================================================
— src/main/org/jboss/resource/adapter/mail/inflow/MailFolder.java (revision 60713)
+++ src/main/org/jboss/resource/adapter/mail/inflow/MailFolder.java (working copy)
@@ -64,7 +64,8 @@
throws Exception
{
// Get a session object

• session = Session.getDefaultInstance(sessionProps);
  + session = Session.getInstance(sessionProps);
  +
  // Get a store object
  store = session.getStore();
  store.connect(mailServer, userName, password);

I believe that using getDefaultSession() was incorrect, because we don't need defaul we need our session =)

Please plan this to be fixed.