java could not send mail

We have a JNLP app that is supposed to send mail after a file is uploaded.  For most of our users, this works fine.  However, there are 4 or 5 users where the email is not sent.  I added debugging and for those that could not send mail, this message is logged:

Could not sendMail: javax.mail.SendFailedException: Send failed;
  nested exception is:
      javax.mail.MessagingException: Connection refused: connect;
  nested exception is:
      java.net.ConnectException: Connection refused: connect

As I said, this is working for most of the users.  Does this message suggest an error on the client side?  Perhaps firewall on their end?  The files they are uploading are always successful, just not the email.  Also, I should point out that for one user, email worked in the past but now it does not.  I had them clear their java cache but still no luck.

What else should I check?  Any settings?
mock5cAsked:
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Sharon SethCommented:
Are these users able to send mails using their regular email clients? I assume mails are sent from individual email accounts ? or is it from a common account?
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CEHJCommented:
To which mail server are they trying to connect in order to send the mail?
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mock5cAuthor Commented:
I asked one of the users to send email to he address which is used by the application.  They sent using their normal email client and it was successful.

This app uses javax.mail and it connects to the the smtp host that is at our institution.
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Sharon SethCommented:
Probably the firewall's program control on the impacted users' machines does not allow java.exe to speak to the smtp port ? It might be worth checking.
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mock5cAuthor Commented:
Do you know of an easy way to verify if the machine's firewall is blocking java.exe from communicating with the smtp port?  Perhaps ask them to run a simple command?  I'm not exactly dealing with the most computer savvy people in the world.
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CEHJCommented:
I asked one of the users to send email to he address which is used by the application.  They sent using their normal email client and it was successful.
I would start by inspecting the headers of the successful message. You need to look at the Java in that light
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Sharon SethCommented:
Check to see if java.exe is on the irewall's blocked list for network communications
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