configuring sendmail server to restrict the incoming mail,unsubscribed by the clients

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http://www.linuxquestions.org – hello friends.... now i m in job...and the task is that.. i m configuring the sendmail server and i want that if any of my mail clients marked the mail as a untrusted mail . then after, the particular client will not get mail from that mail id. for example:- if there are 2 sendmail users named user1, user2 with email id's [email protected], [email protected] respectively. now if [email protected] sends an email to [email protected] and then user2 marks the mail of user1 as an untrusted mail. then after, user2 should no longer get mails from [email protected] anymore... now please hel (HowTos)