Remember to modify /etc/rc.conf, change entry

clamsmtpd_smtp="mysmtp.com:25"

to your smtp server setting, and set

clamsmtpd_enable="YES"

to enable clamsmtpd.

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

content_filter = scan:127.0.0.1:10026
receive_override_options = no_address_mappings

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