You’re creating a string representation of an array, when you need an actual array!
$blogusers = get_users();
$options = array();
foreach ( $blogusers as $user ) {
$options[ $user->user_email ] = $user->user_email; // No need to pass through __(), don't translate emails!
}
And then pass it to your options
argument:
$this->form_fields = array(
'sec_r2_mail' => array(
'options' => $options,
// Other arguments
),
);