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The issue of relaying creating problems has been around for a
long time. spammers (people who send unwanted email)
frequently use it to hide who they are or where their mail is
coming from.
Picture the following:
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ISP #1 has an SMTP server with an open relay.
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spammers use it to send mail to ISP #2.
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ISP 2's users complain, either to ISP 1 directly or through
their own ISP 2.
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ISP 2 decides to avoid such hassles by blocking all mail
passing through ISP 1.
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ISP 1's customers then complaint to ISP 1 about being
unable to send mail to ISP 2 users.
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ISP 1 ends up being forced to close down the open relay.
ISP 1 is the University, and ISP 2 is a service for someone
you want to send mail to. You get the picture.
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