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We recently got an email from an outside server that had a spoofed "From:" header that was the email address of one of our users. Is there a way to have sendmail reject emails coming in from an outside network that claim to be a specfic user, but still allow that user to send mail from our own servers? How can I set this up?

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You can use SPF or DomainKeys Identified Mail. And configure sendmail to verify SPF and DomainKeys. This will reduce some of your received spam too.

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