This DOES NOT work:

// message and recipient are both strings
function sendEmail($message, $recipient)
{
  require_once(e_HANDLER . 'mail_manager_class.php');

  $mailer = new e107MailManager;
  $mailer->Mailer = 'php';

  // Create the mail body
  $mailData = array(
    'mail_total_count'     => 1,
    'mail_content_status'  => MAIL_STATUS_TEMP,
    'mail_create_app'      => 'core',
    'mail_title'           => 'NOTIFY',
    'mail_subject'         => 'Test email',
    'mail_sender_email'    => '[email protected]',
    'mail_sender_name'     => 'Me',
    'mail_notify_complete' => 0,
    'mail_body'            => $message,
    'template'             => 'notify',
    'mail_send_style'      => 'notify',
  );
  // pass recipient email address as a string
  $mailer->sendEmails('notify', $mailData, $recipient);
}

This DOES work:

function sendEmail($message, $recipient)
{
  require_once(e_HANDLER . 'mail_manager_class.php');

  $mailer = new e107MailManager;
  $mailer->Mailer = 'php';

  // Create the mail body
  $mailData = array(
    'mail_total_count'     => 1,
    'mail_content_status'  => MAIL_STATUS_TEMP,
    'mail_create_app'      => 'core',
    'mail_title'           => 'NOTIFY',
    'mail_subject'         => 'Test email',
    'mail_sender_email'    => '[email protected]',
    'mail_sender_name'     => 'Me',
    'mail_notify_complete' => 0,
    'mail_body'            => $message,
    'template'             => 'notify',
    'mail_send_style'      => 'notify',
  );

  $recipientData = array(array('mail_recipient_email' => $recipient, 'mail_recipient_name' => $recipient));

  $mailer->sendEmails('notify', $mailData, $recipientData);
}