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Where should I look for this.

Could you please provide some reference, web links,or books for this kind of LAN server scenarios. So I can go through that, and can understand more comlex server installing scenarios and more practical scenarios.
Thanks for your relevant help.

In clustering case all three mail server will behave as a single server,means any mail server is able to receive any the same mail. But we dont want that.

We are takeing name for each mail server. eg. mail1.myexaple.no-ip.org, mail2.myexaple.no-ip.org,mail3.myexaple.no-ip.org.

mailserver1 has users u1@mail1.myexaple.no-ip.org,u2@mail1.myexaple.no-ip.org.

This is the scenario we want to achieve.
mailserver2 has users u1@mail2.myexaple.no-ip.org, u2@mail2.myexaple.no-ip.org

mailserver3 has users u1@mail3.myexaple.no-ip.org,u2@mail3.myexaple.no-ip.org

I am not much clear about domain. But as per my understanding here in diagram public IP 8.3.2.45 which is resolved by second level dns server is representing domain.Please Let me know if my understanding is correct here.
So they are in same domain. If they are in the same domain, then can we use sub domain for individual mail servers?

what is Round robin clustering here?

Could you please explain in the network diagram I previously attached to get these server accesed from internet. And if anything is need to changed in the diagram or needed to add in the diagram.

OK I am taking one diagram here having 3 mail servers. Now my objective is to access all three servers from internet.
Now MAU (eg. thunderbird or outlook) will send mail to SMTP server. SMTP server will relay mail to MTA. Now the MTA will resolve ip of donmain myexaple.no-ip.org ie 8.3.2.45 and in last will send mail to  IP 8.3.2.45 on port 25. And this IP 8.3.2.45 is a router's WAN port's Public IP. and this ip + port ie. 8.3.2.45:25 is mapped to ip+port 192.168.1.20:25. So this mail will reach to 192.168.1.20 mail server.
So In this case we are not able to access mail server 192.168.1.30 and 192.168.1.40.

And the problem here is that MTAs always send mail on port 25 in case of SMTP not SMTPs.
If MTA is knowing that router is configured on port 25025 and sending mail to ip 8.3.2.45 on port 25025 then mail will reach to this mail server192.168.1.30. but practically MTAs don't know what port are configured on router to reach to 192.168.1.30 or to 192.16.8.1.40.
So this is the problem here.
Now If we are taking a dns server inside lan, I am not getting what mx entries  this dns server will have, and who is making dns query to this dns server. And how this LAN DNS is going to help.
Thanks

Screenshot file attached is not opening, so attaching doc file having network diagram for above description.

For the comment 'external DNS, the mailserver record will point to the public IP address', I have one query, which device like router, firewall is assigned this public IP which is pointed by mx record in dns, or this IP is used only for forwarding without assigning to any device.

First I thank you for your quick reply. I am taking a small office network. It has a email and a web server iside the lan. Lan is connected to lan port of router. All lan machins inside lan are configured with private IP. Routers WAN port is configured on public IP. router will do port forwardind for any request coming on this public ip  on port 25 or 80. And it will reach to that mail/web server. In this scenerio We c
annot have two or more mail/web servers inside the lan. And why do we need DNS server in our lan if we have a registered domain.

Where a mail and web server is kept. Is it placed inside lan confidured with private ip. Or it should be configured with public ip.