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/ Second row only neccessary if password is required.

relayhost =    <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 

That's it!