Really simple Postfix installation
Install postfix. Follow instructions to replace sendmail with postfix.
pkg_add postfix
Allow mail to be sent to ISP's mail server. Edit /etc/postfix/main.cf. Second row only neccessary if password is required.
relayhost = smtp.isp.net
smtp.isp.net <user>:<password>
Link to sendmail default aliases file.
ln -s /etc/mail/aliases /etc/aliases
Make sure local mail to root is sent locally. Edit /etc/aliases.
root: root@localhost
Check that it works
echo "Test of external mail from solo" | mail -s "External mail test from solo" <your-mail-address>
echo "Test of local mail to root" | mail -s "Local mail test from solo" root
mail
That's it!