EMail

Sendmail/postfix
Mail server like Microsoft Exchange Server. It supports SMTP to accept from and deliver to other servers. Postfix is easier to set up.

If your ISP blocks the SMTP port (25), you can not send/receive. But you can use postfix relayhost to send through a remote SMTP server (from your ISP). Instructions can be found here.

IMAP/POP
Protocols allow user to download email from a remote server. These are daemon as well.

Text Based Mail User Agent (MUA)
The two most popular ones are pine (alpine) or mutt (new elm). pine/alpine does not come with Centos. Alpine comes with at least Fedora 9.