Update Postfix configuration instructions 

Fixes #1139.
Where we can it would be good to have these copy and pastable, e.g:

cat > /etc/postfix/recipients <<EOF
/^foi.*/                this-is-ignored
/^postmaster@/          this-is-ignored
/^user-support@/        this-is-ignored
/^team@/                this-is-ignored
EOF