MX records points to an IP address that is not a public IP

[myx9]

My intern ip addresses are private. 192.168.x.x,
and MX records from my server points to intern IP address that are not public IP.
for ex:
"Received: from pias [192.168.2.210] for mail-local (HELO pias)
via SMTP; Mon, 23 Jan 2006 16:26:51 +0200"

How to change my MX records ?

[Roxoff]

Are you receiving and sending mail ok? My mailserver sits behind my firewall, the firewall has the public IP address assigned to it, and my mailserver uses a private IP. When I send emails, the sent address looks like this:

Received: from [192.168.250.254] (woody.roxoff.org [192.168.250.254])

and I've never had any problems with it.