Sender Policy Framework
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Sender Policy Framework (SPF) is a DNS-based method of authenticating which eMail servers are authorized to send messages from the eMail sender's internet domain name.

This technology helps to prevent eMail envelope forgery.  (Technical note:  The SPF mechanism applies to the sender's eMail address as it's presented in the "MAIL FROM:" command during the SMTP envelope stage; it does not operate on the "From:" SMTP header that is normally presented on-screen along with each eMail message.)

The SPF checking tool is available, here:

More tools are available in the "Tools" section of this web site, including...