install 5.5 exchange on 2000 server

I tried to install exchange 5.5 on a stand alone windows 2000 sever, that was not on the domain yet. It was configure with the DNS ip and has a static ip address. It just is not on the domain yet, as I was afraid what might happen if I loaded it on the production servers yet. I am not the senior guy. It gave me an error that it would not work with the present system. Can 1. I put the server on the domain then load exchange 5.5 and since there is no MX record , my guess is it will not be seen by mail or the other SMTP server. My plan is to make this a second SMTP gateway server in case the other one fails, as it is old? 2. why is it not able to install?