in my dns server i have a domain example.com . i have subdomains under it , like server.example.com , login.example.com etc . the zone file is like this :

IN NS example.com.
server IN A 192.168.1.2
mail IN A 192.168.1.2

when i dig them there appears to be no problem.
in the web server conf file i used two different document roots for the subdomains.the sample code is like this :

<VirtualHost server.example.com>
ServerName server.example.com
DocumentRoot /var/www/
</VirtualHost>
<VirtualHost mail.example.com>
ServerName mail.example.com
DocumentRoot /xyz/
</VirtualHost>

if i browse for server.example.com iam getting the desired html file , but if i browse for mail.example.com the http test page is being displayed .Please help me solve the problem.

This is how I do it on my server:
I use Dynamic DNS, but that won't matter if dig works fine.

ServerName 155.143.61.52
NameVirtualHost *:80

<VirtualHost *:80>
ServerName prestigespas.homeip.net
ServerAlias homeip.net *.prestigespas.homeip.net
DocumentRoot /var/www/prestige
</VirtualHost>

<VirtualHost *:80>
ServerName australianile.homeip.net
ServerAlias homeip.net *.australianile.homeip.net
DocumentRoot /var/www/ane
</VirtualHost>