你可以做的一件事就是过滤$email_to 如果收件人已登录，则返回空字符串。这边wp_mail() 将无法发送消息并返回false。将以下内容添加到主题functions.php 或至bp-custom.php 文件：

add_filter(\'messages_notification_new_message_to\', \'disable_loggedin_email_notification\');
function disable_loggedin_email_notification($email_to) {
    $user = get_user_by(\'email\',$email_to);
    if (bp_has_members("type=online&include=$user->ID")) {
        $email_to = \'\';
    }
    return $email_to;
}

add_filter(\'messages_notification_new_message_to\', \'disable_loggedin_email_notification\');

function disable_loggedin_email_notification($email_to) {
    $users = get_users(array(
        \'search\' => $email_to
    ));
    $ids = array();
    foreach ($users as $user) {
        $ids[] = $user->ID;
    }
    $ids = implode(\',\', $ids);
    if (bp_has_members("type=online&include=$ids")) {
        $email_to = \'\';
    }
    return $email_to;
}