I have CentOS6 and installed LAMP server.

The server won't send emails, althrough it says PHPMAIL is installed. I checked this using php script:

<?php

if ( function_exists( 'mail' ) )
{
    echo 'mail() is available';
}
else
{
    echo 'mail() has been disabled';
}  

?>

What's causing this? I have the same server on another hosting provider and it sends email normally. What settings to check?

Found it out!

Perhaps Linode CentOS image comes without (!) SENDMAIL installed, therefore PHPMAIL was not working.

I installed SENDMAIL with

yum install sendmail

and rebooted Linode. After that, PHPMAIL function started to work.