Postfix Install
To create a functional SMTP server, first you need to install Postfix by running
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yum install postfix
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Check to make sure that your hostname also has your fully qualified domain name.
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echo $HOSTNAME
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If it does not have your domain, then you must add it to the configuration file.
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myhostname = smtp.example.com
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You must also make your SMTP server listen on an interface besides the localhost. Uncomment:
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inter_interfaces = localhost
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To:
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inet_interfaces = all
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Now edit the configuration to allow trusted networks to relay emails. In most situations uncommenting:
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mynetworks_style = subnet
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should be sufficient, however if you are allowing a larger network or deal with multiple networks then manually add the networks that will be trusted.
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mynetworks = 127.0.0.0/8, 192.168.1.0/24
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ClamAV
Download and install clamav, clamav-db, clamd, clamav-milter from http://packages.sw.be/clamav/

Edit the init scripts to allow Postfix to read the clamav-milter socket. Add the following lines in the start, stop, and restart case statements.
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chmod 0775 /var/clamav/clmilter.socket
chown clamav.postfix /var/clamav/clmilter.socket
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If these settings are not set, Postfix will not be able to correctly communicate with the ClamAV milter and will receive a
warning: connect to Milter service unix:/var/clama/clmilter.socket: Permission denied
in the mail log

Now add:
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smtpd_milters = unix:/var/clamav/clmilter.socket
non_smtpd_milters = unix:/var/clamav/clmilter.socket
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to the bottom of main.cf and restart the Postfix service.

Now add freshclam to the cron to get automatic updates and everything should be all set.