|@StuartS: Sendmail both sends and receives mail (despite it's name). It's just an SMTP server which send mail to, and receives mail from, other SMTP servers.|
It's true that it's not a mail client, but that's another matter.
@KevinS: First things first; does your DNS server have an MX record pointing to the server hosting CentOS? This is the first essential to receive mail, else other mail servers won't be able to find it.