A mail server is only required if you're looking to receive mail. Even this can be avoided by using a tool such as
fetchmail (for receiving).
In general it's probably best not to setup a mail server, given the extra work involved in having to maintain it. If you really want to though, I've conveniently put together a tutorial titled: How to Setup a Mail Server on CentOS 5 on my blog. It covers how to setup a sendmail based mail server with all the add-ons (spamassassin, virus checking, etc.).
Usually all that's needed to send mail is the ability to queue messages, and then periodically deliver them. What this means is that you can setup something like this, using sendmail.
Create the file
Then run this command:
$ m4 /etc/mail/sendmail.mc > /etc/mail/sendmail.cf
excerpt from the Fedora Project
The following article explains the setup of sendmail for forwarding
mails to your ISP's smtp server. It is assumed that you have sendmail
up-to-date and configured properly. Note: this setup does not work
properly for smtp-server using SASL (e.g. like smtp.pobox.com) Most of
the commands must be executed with the corresponding rights (using
Create/Edit the file
AuthInfo:<your-smtp-server> "U:<your-smtp-user>" "P:<your-smtp-password>" "M:DIGEST-MD5"
Set it's permissions:
$ chmod 600 /etc/mail/authinfo
Rebuild the authinfo file:
$ makemap hash /etc/mail/authinfo < /etc/mail/authinfo
-or on Red Hat based systems-
Restart the service:
$ /sbin/service sendmail restart