I know everyone else has said it but postfix is a great replacement for sendmail.
Another tool I've found that I like is ssmtp. It's not a replacement for sendmail/postfix by any stretch but if you want a simple down & dirty tool to send email from an internal server to your main email server it's good. I use it on a server at home and on test rigs at work for emailing results of cron jobs to my own account. Don't know if it's available in yum as I haven't used it on a CentOS box yet.