Email-related daemons can be put into two categories: mail transport agents (MTAs), which send new email messages to their destination machines, and mail delivery agents (MDAs), which send mail that's landed in a user's mailbox to that user's personal computer.
A mail transport agent sends email to other computers, most often via the SMTP protocol. Mac OS X ships with Postfix, an improved alternative to the more common sendmail program that shipped with versions of Mac OS X before Panther.
Run Postfix only if you need to provide mail-sending services to yourself or your network. You don't need to run this service to simply send email as long as there is an SMTP server that will accept connections from your machine; most ISPs provide mail services on their own servers, for example. Try sending through mail.yourispname.com and see.
You can configure Postfix to work in two ways on your machine. The first, as a local mailer, allows you to send and receive local messages, as well as send messages to external Internet addresses. This mode is useful for receiving the regular cron reports that get sent to root, for allowing scripts to send mail, and for sending quick messages from the command line using the mail command.
Postfix can also run as a standalone mail server, able to exchange mail with other servers on the Internet. Even if you don't need to run your own full-fledged mail server, this mode lets you use your regular GUI ...