Send Mail error On freebsd,sendmail

[eluser]whidbey[/eluser]
I have installed sendmail in freebsd system.
my CI controler code is:
$this->email->from('[email protected]', 'whidbey');
$this->email->to('[email protected]');
$this->email->subject('Email Test');
$this->email->message('Testing the email class.');
$this->email->send();
echo $this->email->print_debugger();

I got the error:

Unable to send email using PHP mail(). Your server might not be configured to send mail using this method.

From: "whidbey"
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what can I do ?thank you