Another alternative to email notification is to use Postfix instead of Sendmail because Sendmail is not as secure besides, using Postfix is a lot easier and it is more secure than Sendmail.
If you want to proceed with it, you have to remove Sendmail and replace it with Postfix, using these 2 commands:
rpm -e –nodeps sendmail
yum -y install postfix
Following this, edit your /etc/postfix/main.cf, (use your preffered editor and add your preffered SMTP) and add:
relayhost = smtp.your-smtp.com
Replace smtp.your-smtp.com with your own smtp server.
You should also edit /etc/asterisk/vm_general.inc and set the “serveremail” parameter to your real email address. This is to avoid messages with invalid from-addresses floating aimlessly in the Internet.
Then load Postfix.
service postfix reload
Postfix will now send all outgoing email to your ISP’s mail server.