I have a backend that used to be my front end. Now my front end is an nginx proxy server. It just serves as an http and an IMAP/SMTP proxy. The backend services apache,mysql,dovecot/postfix. Since I defined the nameservers to this box on the registrar, do I now define the nameservers to the proxy box? It is located in a different geographical location? If so, what hostname should the backend have? The proxy needs to resolve the MX. Thanks.
By defining nameservers at registrar to this box I assume that you are referring to glue records pointing to the IP(s) of that server. It means that you also have a DNS server running on that box, such as BIND or DNSMASQ. You would need to move that service to the new box.