Very Computer

This has been driving me nutty. I decided to setup a small business with
linux as a file/printer/e-mail/internet server (2 offices, about 10 clients
at each)
I can do all of the rest (file,printing, and internet access) but I am
having a hell of a time understanding linux (unix) e-mail. I have searched
countless how to's but just want to know one thing. How do I do the
following:
Allow 2 offices to send and receive e-mail to each other
Allow each office to send and receive e-mail locally (don't go out to the
internet when you want to send something to the guy in the next cube)
Have a dial up connection retrieve e-mail from an internet POP server, and
deliver it a win95 client when the win95 client requests it.
I'm assuming i need sendmail (or qmail), a pop server, and a ppp connection.
DO i need a DNS server or can I just resolve through hosts file?
Can someone please point me to a good source on the subject of e-mail, or
offer some advice.
Thank You