EBS 11i/R12 -- Workflow mails are not sent --"undelivered","undeliverable"

If you configured smtp and imap, and if your emails are not sent to the receivers, then you may be facing this problem.

Check your Notification mailer logs... If you see your Imap thread(inbound) is trying to process an email which has a undeliverable or mesage delayed subject, then you are facing this issue ..
Here is an example;

Customer complains about the notification emails are not sent to the receivers.
We start our investigation and see the messages are sent in the notification mailer's log file
WFMLRSVC-371100-10006 : oracle
.apps.fnd.wf.mailer.SMTPMessageHandler.sendMessages(String)]:Updating notification {1532500} status {SENT} {} {} ..
So everyting seems proper.

After analyzing the log, we see an IMAP related line, which seems not related with our issue in first sight, because the problem should be in the SMTP, so why looking IMAP logs?
WFMLRSVC-371100-10006 : oracle.apps.fnd.wf.mailer.IMAPResponseHandler.processSingleMessage(Message)]:Processing Message SUBJECT[Teslim Edilmedi: Closed : ERMAN --> "ERMANANAAA"]

But, there is an imporant clue here.. Even if it is related with IMAP, we see that the mail has a "Teslim Edilemedi" tag in front of its subject. Teslim Edilemedi means undelivered, undeliverable in English.

So , this means our smtp request was rejected by the mail server.. 

Normally when an email is rejected, the mail server sends a notification mail for giving the information to the sender that the mail cant be delivered. So in this time, this undeliverable information is sent to the IMAP account. This reveals the problem , as the email cant be sent even if it seems SENT in the log file.

That 's it. Your smtp server can not deliver the emails to the receivers. Our part is done.   At this point, we have already exposed the problem.. It should be a problem with accounts. a disabled account in a mail group or something like that..

Now it is time to let the Mail Server Admins work :)