/* ================================================================= */
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. Add the following in the config file or 
comment them out.

OutAddress: 127.0.0.1:10026
Listen: 127.0.0.1:10025

To work with postfix, put following lines in Postfix' main.cf:

content_filter = scan:127.0.0.1:10025
receive_override_options = no_address_mappings

And modify master.cf to active it. See also
http://memberwebs.com/nielsen/software/clamsmtp/postfix.html
/* ================================================================= */