Hello usmana,
To set up emailing you need an email server software installed on your droplet.
Here are two guides you can follow to setup emailing on your droplet.
So depending on your anticipated usage, you can go through these links to learn more about emailing and how to properly setup emailing to fit your needs.
For more on learning to do things with droplets use the search box or click on the tutorials link at the top to see more tutorials on assorted topics.

1. https://www.digitalocean.com/community/tutorials/how-to-install-postfix-on-centos-6

2. https://www.digitalocean.com/community/tutorials/how-to-configure-a-mail-server-using-postfix-dovecot-mysql-and-spamassasin

Hope you find these useful.
Cheers.

I have installed WHM and created cpanel for each domain. I created emails using cpanel. I could send emails to any from those emails but can not receive emails from other mailing addresses like gmail or hotmail.

Delivery to the following recipient failed permanently:

     usman@mydomain.com

Technical details of permanent failure:
Google tried to deliver your message, but it was rejected by the server for the recipient domain mydomain.com by aspmx.l.google.com. [2a00:1450:4010:c07::1a].

The error that the other server returned was:
550-5.1.1 The email account that you tried to reach does not exist. Please try
550-5.1.1 double-checking the recipient's email address for typos or
550-5.1.1 unnecessary spaces. Learn more at
550 5.1.1 http://support.google.com/mail/bin/answer.py?answer=6596 pk5si17857602lbb.60 - gsmtp

Here, mydomain.com is just a example. PS. There is my domain and its working perfectly.

What will be perfect solution?