Some users of trixbox have port 25 blocked by their ISP’s this blocks out sendmail. See the following article http://dumbme.voipeye.com.au/trixbox/trixbox_without_tears.pdf
Remove the entry in the etc/hosts file referencing your server.
Remove Sendmail and Install postfix
rpm -e --nodeps sendmail
yum -y install postfix
Following this, edit your /etc/postfix/main.cf, (use nano) and add:
relayhost = smtp.example.com
Replace smtp.example.com with your own smtp server. E.g. if your belong to Demon
it should be:
relayhost = post.demon.net
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