i know this is not the right place to raise this question but i am sure most of the people must be face this problem and might someone can suggest me any solution.

For our client we provide MPLS ( private network ) and as well as internet network to access our applications and mails. we were doing testing for network redunancy to access our mail server means if MPLS gets down then client should able to access mail through broadband and vise versa..

Now the problem is user configuring outlook with domain name like mail.example.com. Now question raise how to do the ip configuration for both connectivity. so we have put entry in hosts file of client's windows xp m/c for both ip..for example :
< private ip address of mail server > mail.example.com
< public ip address of mail server > mail.example.com
now when user access the outlook its goes through private network ..we have checked through netstat and when we disconnect the MPLS connectivity then user has to comment the first entry then only request goes through public ip as hosts file is not intelligent to check the second entry if first one doesnt work.
Here below 2 disadvantages are :
1. user has to manually comment the first entry when MPLS goes down
2. my private ip address of mail server will get expose to users which i dont want.

So is there any other way to do the settings ? or we need to do any configuration router / firewall end ??
It wil be great If any one suggest me anything on this.