PHP 5.1.6 - PHP Fatal error: Call to undefined function mail()

Is anyone aware of a bug of PHP 5.1.6 causing:

Fatal error: Call开发者_运维知识库 to undefined function mail() ?

I have the same code running smoothly in PHP 5.2.0 but on this server with PHP 5.1.6 it is nondeterministic whether the mail function is defined or not. I am sure the problem is not in the code itself because simply running this script:

// PHP 5.1.6
<?php 
if (function_exists('mail')){
    print 'mail is defined';
}
else{
    print 'mail is undefined';
}
?>

... php prints sometimes "mail is defined" and sometimes "mail is undefined".