Regarding Mail Server.. which is Mail Services or Exchange?

Hello m network administrator in health care company, i want to configure the mail server on the network, as we have 1 DC running on window 2003 server and having 1 ADSL modem for internet connection. i have 10 to 15 cleints that are using the winXP. now they all want their email address, so plz suggest me wot will i have to install, either i have to go for email services or exchnage server.
also they want that other domains can send the emails to them.
do the needfull.