How to Setup an Internal Mail Server with Linux?

Hello there..
I am a newbie to Linux. I want to know how to setup an internal mail server in linux. I know it can be done with sendmail, but don't know how it can be done.

Every user on my network has got an email account at our mail server which is being hosted and managed by our ISP. Users use POP3 clients such as Outlook Express to access their mails on the server.

Now I want to setup a mail server which will automatically detect mails for local recipients and will transfer the message locally (no need to send it to the mail server at ISP). And also when a mail is destined for outside, it will forward it to the mail server with ISP.

Let me know how I can accomplish this.

Thanks in Advance.