Sending email using function drupal_mail(). All you need to do is install and configure SMTP module and then add the given code in your custom module.

function sendemail(){        
      $to = ' xyz@gmail.com ';
      $from = ' abc@gmail.com ';
      $subject = ' Subject of your mail ';
      $msg = " Hello dear,how are you ? ";
      mymodule_mail($from,$to, $subject, $message); 
}

/**
* * Implementation of hook_mail().
*/
    function mymodule_mail($from, $to, $subject, $message) {
      $my_module = ' mymodule ';
      $my_mail_token = microtime();

      $message = array(
        'id' => $my_module . '_' . $my_mail_token,
        'to' => $to,
        'subject' => $subject,
        'body' => array($message),
        'headers' => array(
          'From' => $from, 
          'Sender' => $from, 
          'Return-Path' => $from,
        'MIME-Version' => '1.0',
        'Content-Type' => 'text/html; charset=UTF-8; format=flowed',
        'Content-Transfer-Encoding' => '8Bit',
        'X-Mailer' => 'Drupal'

        ),
      );
      $system = drupal_mail_system($my_module, $my_mail_token);
      $message = $system->format($message);
      $system->mail($message);
    }