Security implications re: NFS from public server to private

[Dick Hertz]

Okay, I have a four-legged firewall setup. fxp0 is the external interface, fxp1 the clients, sis0 the private server, and sis1 the public. The public server is going to be running qmail, courier-imap, and bind9. The private server has my documents, mp3s, etc. on it. It is not presently open to the outside world save for the VPN (openvpn).

In order for my incoming email to not "live" on the public server, AFAIK, it will need to mount the home directories on the private one via NFS or somesuch. This defeats the purpose of having public/private servers, no? What are the security implications of either choice?

Any help would be appreciated.