Mail Getting Bounced

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http://www.linuxquestions.org – Hi Our mail server is Postfix 2.2 running on CentOS 4.7. It was working fine since last few years. Suddenly no user could send mail using this server. The sent message is getting bounced with the following message ( in postfix logs ) : 550 Administrative prohibition (in reply to RCPT TO command) On speaking to our mail service provider, they say that they have added some authentication and needs to be done on our internal mail server. Can any one help in getting the same sorted out ? Thanking in advance ra (HowTos)