New linode user need help with SMTP accounts and installing webmail

Hello,

I am using Linode1024 server with,

Linux Operating: Ubuntu 14.04 LTS

HTTP Server: Nginx

SMTP server: Sendmail

Mail Transfer Agent (MTA): Postfix

PHP: 5.5.9

MySQL: 5.5

PHP's mail function is working fine on my website.

1) I want to use different email address like contact@mysite.com, admin@mysite.com with passwords. Where should I setup those emails addresses?

2) I also want the functionality to receive emails from my client.

3) I want to use "squirrelmail" webmail for receiving and sending emails, will it work on my system? If not, please suggest good webmail.

Thanks for the help.

Ankur