Email impersonating

Hi,
With Java Mail I can send an email as any one else by setting from address.
How can this be stopped? Digital signatures? How?
Or at least the recieving person should be alerted that this email is not authenticated.
Thanks

I found this
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Q: How do I send secure email using the JavaMail APIs?
A: The JavaMail APIs currently have no support for sending or receiving secure email. The architecture of the JavaMail APIs allows such support to be easily added later, by us or by third parties. Information on the current Email security standards (S/MIME and PGP) can be found at http://www.imc.org/smime-pgpmime.html. Please browse our Third Party Products page for solutions from other vendors.
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Just about any email client can set the From: header any way it pleases. That's not a feature specific to JavaMail. If you are suspicious of the sender, you'll need to look at the other header information to see where the message came from.