sendmail relay

["Yong-Ming P.Hua" <yhua@example.com>]

Hi,
   One of my servers got a message from orbs.org. The server allows replay so
they suggest us to stop relay...

My question.

   We want our clients (students and teachers) get mails via pop from our
servers,  from outside the campus, send mails via ISP from our servers.

if sender is (mail from:sender....) hasn't got our domain name, relay is
blocked. But if sender has our domain name like
mail from:nobody@example.com

relay is accepted in a helo protocol.(any user name is accepted of course).

Any idea to block ANY kind of relay from outside? (though it is easy to limit
mail activity only within campus or to certain domains, but in that case you
cannot use smtp from an unexpected place....)
I am using 8.9.3/3.7Wpl2.

Thanks in advance

Yong-Ming

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