mail servers

a. i want to setup a mail server for an existing site.
b. do i hire someone’s mail server software?
c. do i put the mail server software on the same system as my web site?
d. who all offer mail servers?
e. how much traffic does a mail server hosting provider allow free of cost and how much do they charge after the limit is reached.

in all, i need a white paper on mail servers, their working and mail server providers. please respond at the earliest.

Srivas