The BLFS documentation for installing postfix is at http://linuxfromscratch.org/blfs/view/stable/server/postfix.html. Please make sure that your CCARGS includes the TLS requirements, and your AUXLIBS specifies both libssl and libcrypto. Don't include any of the SASL options.
Use the boot script package to make your postfix start at each reboot.
I will test local mail delivery to your "testuser" account much like we did with the postfix install to your Debian machine.