How are the mail servers connected?? via private line ie T1/DS3?  What has changed since then?  Do the logs on either mail server show anything suspicious?  Please provide a little more detail as to what is exactly failing? The more information you can provide us the better we will be able to help you!

Well as I had specified previously that one of the mail server is in 192.162.0.0 series through a DNS Server and this is in local LAN and the other is in another location but in Intranet only through E1 line and connected through a Router in the IP series 172.16.0.0 through a gateway. Hence I had configured the system with Dual IP address for accessing both mail servers. The moment I remove either of one IP entry then the same will work without any problem and the moment i configure both IP say after some 2-3 mts it will not access both the servers but can ping both. I had done an netstat -an then it shows that it is opening ports to connect to microsoft-ds.com site. Pl reply back.

Ok, I will explain properly. We have two mail servers one in LAN and another in Intranet (in Remote Location). These two mail servers are working perfectly if contacted by our LAN users individually i.e.
Scenario 1-
If I want to use the Mailserver(It is not Exchange Server-Local SW) of Intranet then I have to change my IP address to that series and access by Browser then I get connected to that mail server.
Scenario 2-
If I want to use the local LAN MailServer configured through Exchange Server ,once again I had to change back to my Local LAN IP series and get access by outlook, then I get Connected to this Exchange Server.

But I want to access both without changing the IP Series i.e. by adding both IP address in the TCP/IP configuration of LAN settings.

Previously, we had encountered Fujibacks virus in four systems, and in these four systems we removed the virus and re-formatted these systems. For 15 days these system started working perfectly and then suddenly seized working when given dual IP, Checked for Virus but no luck. Pl Reply and Advice.

If i'm understanding you right why are you configuring a client with 2 different ip addresses?  If the local subnet for the client is 192.162.0.0 and the remote is 172.16.0.0, you don't need an IP in both subnets.  Just a route to get there...if you need to hit a different router then your default gateway to reach that subnet adding a static route may be what you need to do.

Nelson,

how to add the static route in the router will vary depending on device/manufacturer. Be aware that you will need a route at both ends to allow traffic to return.

However, as the 2nd mail server is on a remote site do you have an point to point connectivity between the two sites? As both ranges are internal rfc 1918 addresses these aren't routable over public infrastructure. If there is no direct connectivity you will need a site-to-site VPN to link the two LANs.

Hope this helps,
RR

(P.S. Still not clear how this relates to ethical hacking though....)