Need help for mail servers solution !!!

[HaoDr]

Hi all,

I intend to design a mail servers solution as follow,( if you find out something wrong, please point out and correct them for me):

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Internet
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Relay mail server (SMTP / Sendmail )
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------------- Private Netwrok ----------------------
Mail server 1 (smtp/pop3) Mail server 2(smtp/pop3)
/|\ /|user 1(Microsoft Outlook/win98) ... user n(Outlook/Win98)
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*- Relay mail server has the full qualify domain name: smtp.hcmu.edu, IP address: 203.162.20.10 /
24
- mail server 1 has full qualify domain name: mail.dientoan.hcmu.edu, and IP address: 172.16.10.100 / 24
- mail server 2 has full qualify domain name : mail.vatly.hcmu.edu, IP address: 172.16.20.100 / 24

* Setting up in DNS server like that:
dientoan.hcmu.edu MX 10 mail.dientoan.hcmu.edu
dientoan.hcmu.edu MX 20 smtp.hcmu.edu

vatly.hcmu.edu MX 10 mail.vatly.hcmu.edu
vatly.hcmu.edu MX 20 smtp.hcmu.edu

* In the Relay mail server ( smtp.hcmu.edu ) , we configure it to allow relay domains:
- dientoan.hcmu.edu
- vatly.hcmu.edu
- 172.16.0.0 / 16

* setup in users (Outlook Express / Win98):
Outoing mail server : mail.dientoan.hcmu.edu / mail.vatly.hcmu.edu
Incoming mail server: mail.dientoan.hcmu.edu / mail.vatly.hcmu.edu

In addition, I am not sure about the using private IP (172.16.10.100 or 172.16.20.100 ) with MX record for public IP of Relay mail server (smtp.hcmu.edu: 203.162.20.10 ) as I have configured in DNS server. What do you think about this setting ? does it work ? Or if you have better solutions please suggest me by the way.

Thanks so much for your help.

Hao Dr