Software

what it appears you are trying to accomplish from your questions is to provide Web Site Services, Name Resolution Services and Email Services and/or Email Fowarding Services. If this is the case, please write a comment to your question.

this is a addendum to my answer to your first question.

Web Browsers assume that when you type in a name your looking for a web site and uses the default port 80, even if you type in just the ip address with no port # specified. That is how using names instead of #'s work. How names to IP addresses are resolved is nothing more than having a A record that says <Mydomain.com> is IP address 100.100.100.100. When a Name Server sends out a Question to the internet asking "WHO IS" <Mydomain.com>your name server [or a name server who is authoritative for your domain name ] responds "<Mydomain.com> is IP address 100.100.100.100. Email works basically the same. A MX record says <@mydomain.com> is IP 100.100.100.100 Port 25 and your email server listening in on port 25 for traffic accepts email send to your IP address. The mail server then reads the header [e.g. the recipient's name] checks it's data base and then stores the mail until the users requests the mail via Outlook or some othermail program.

As long as there are records created to resolve names to IP addresses and email names to IP addresses, and broadcast that to the internet whenever there is an inquiry about "WHO IS" one can "Host" just about anything provided you have the equipment to handle the traffic.

the drawback to hosting email services is that clients have to connect to your mail server to get their mail. The most widely use mail services are SMTP or POP. Both require users to log in and then either get mail via a client program like Hotmail or download it for later reading. Heavy equipment investment. One can provide email fowarding services where users have a Web site like www.<mydomain.com> Email address @<mydomain.com> but want mail addressed to @<mydomain.com> sent to another email address. A MX record is created that basically says, email addresses to @<mydomain.com> is to be sent to @<mydomain"A".com or @<mydomain"B".com>.

Ok. Internal web sites only for clients on the LAN.

This operates basically the same as for the internet except that you don't need to register the domain names with a registrar service. The names one uses are private not public name spaces.
Two Nic's verses one NIC with two IP address is kind of a resource consideration or just a preference based upon how comfortable one is editing the registry settings so http port 80 is bound to the proper IP address so there isn't a conflict.

For DNS name resolution, Technet article 168321 tells you how to create DNS records for either way so that Name to address resolution works for the different IP addresses.

If you are trying to set up a web site for a company that has seperate offices located in different geographical areas, use the hierchey of name spaces with a master site page that links users to the individual sites for their respective office. <mydomain.com> as the master site with links to <site1.mydomain.com> <site 2.mydomain.com> <site3.mydomain.com>. Users just type in <mydomain.com> and then click a hyperling from there to to get to their area's web site page.