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Edit /etc/rc.conf and set

clamsmtpd_enable="YES"

to enable clamsmtpd. 

Also, there is a clamsmtpd.conf-sample in %%PREFIX%%/etc/, 
rename to clamsmtpd.conf, otherwise specify -f option to your
config file. 

To work with postfix, See also
http://memberwebs.com/nielsen/software/clamsmtp/postfix.html
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