Why not open port 110 (or a non-standard port like 8110) to your mail server so your offsite users can get the mail directly from your server?

If you don't have a static IP or domain name you could set up a 'dynamic dns' one.

If you are doing this you could also open an SMTP port so all the mail goes out through your server as well. (would recommend a non-standard port if you are not wanting to receive mail by SMTP) 

Another option is to set up a forward or alias for these users, but it would have to point to a non-local address.

Is it possible to configure the Mercury system to treat certain email addresses of the local domain as non-local and attempt sending them via SMTP Client?

We have installed MercuryD, MercuryC, Pop3 Client/Server system. We are Domain Mailbox system to get emails for all local users.  

However, we have certain users who have emails in same domain, but are not  in side the LAN . We would like such users to be able to communicate with other uses inside the LAN as well.

 How can this be done?

 Any help will be highly appreciated.

Thanks,

Niranjan
 

Generally this cannot be done unless you remove the domain from the domains list and alias all the local user to a local domain.
 

For example you are using the domain "example.com" where some people using the domain example.com are not part of the local system.  You would change the domain entry example.com to something like local.example.com and then alias the localuser@example.com to localuser@local.example.com.  Becomes a bit of a pain in the rear to handle but trying to make more than one server support a local domain is always a problem.