You're welcome.

For each domain name you use, you'll still have to provide some way for other machines out on the Internet to figure out what address they need to send mail messages to.

You can host multiple domains easily enough on your domain name server, you'll just have to define additional zone files in /etc/named.conf and set up them up like you did deepawan.com.

You probably just want to set up MX records that point to deepawan.com, at least until you get ready to set up web services for multiple domain names (which is a whole 'nother topic).

On the sendmail side, simply add more names to /etc/local-host-names. Which makes one wonder why everybody thinks sendmail is so hard to configure!

but still I have to register all those domains with their nameserver as I did for 'deepawan.com'...isn't it?

You'll have to pay a registrar for every domain you are going to use. Most people only need one domain name.

That's right. Thanks a lot.