Moving SMTP to behind firewall

["Budi Febrianto" <bfebrian gmx net>]

Hi,
I'm planning to move smtp server to behind firewall.
SMTP server running RHL 8.0 with Sendmail 8.12.8-1.80, and firewall is also
running RHL 8.0 with IPTables.

Curently the server have public IP with the host name smtp.xxx.com
(registered in public DNS).
The question is, where should I put the smtp.xxx.com? Should I put in the
firewall (as alias), or still in the SMTP server or both?

Thanks