very easy if you use exchange, but not too sure about the built in POP service. it is very basic.
you can try to backup the mails using ntbackup, and restore them on the new server

Thanks!
The environment where I am applying this can't afford MS Exchange Server, or let's say I don't need its features. The POP3 Service is enough. (You know, cost-effective).

Look, I'll give you the full picture, consider this:

Server1: amjad (and so POP3 Server would be amjad too)
POP3 domain: domain1.com
Account: amjad_acc@domain1.com
Outlook Express is configured to use this account. (on a machine in dept1)

Server2: dima
POP3 domain: domain2.com
Account: dima_acc@domain2.com
Outlook Express is configured to use this account (on a machine in dept2)

The message from amjad_acc@domain1.com can't be delivered to dima_acc@domain2.com
The server amjad can't find the server dima and vice versa.

is there any other possible communication between the domains?

Hi Dima!
OK, I included a couple of images to understand the situation better.
I don't know if there are other ways of communication between the domains. The domains are inside POP3 Server and are created using it.
Look at the images. Inside each domain there are mailboxes, and so the

e-mail would be mailboxname@domainname, in my case amjad-acc@domain1.com

As I said, when the domains are in the same server (same computer), there is no problem. You run outlook, type a message, type an address (whether it is in your domain or the other one), and you send the message. Outlook can find domains because it is configured to receive POP3 mail in the Servers tab from AMJAD

The problem is when I want to send an e-mail to another computer on the network that is configured to receive POP3 email from let's say DIMA, do you understand?

Of course the two servers are connected (on the same LAN)

ok.

can you show me the smtp session logs from both domains?

and the nslookups for MX as well

Hi!
I'm sorry I'm late.

I'm not sure of how to do this, or what we can get by this. Isn't what you're asking for related to MS Exchange Server?

Since you go for the simple cheap solution you should be willing to accept simple cheap answers: Why not configure more accounts for Outlook and just switch to the correct account for where you want to send? If it works when both domains are on the same computer using POP then you have already answered your question. It won't work any other way because the solution is limited as such! Probably on purpose so you buy Exchange! You should just keep running the domains on the same machine, deal with a little hassle in Outlook switching accounts or look into spending some money on the appropriate solution. Good luck!

right.
I need the smtp telnet session printout (you should know how to do it since you're administering smtp servers. if not just google "telnet port 25")

and the same with nslookup with set q=mx