Похоже, кто-то использует мой почтовый сервер для рассылки спама. Что я могу сделать, чтобы его обезопасить?

За последние два дня я видел несколько загадочных уведомлений о «отказе» электронной почты в своем почтовом ящике. Ниже вставлено необработанное содержимое одного из них. Обратите внимание, что я подвергал цензуре свои личные данные. В to адрес для всех этих сообщений: kunnu@mydomain.com.

Речь идет о сервере Ubuntu 9.10, на котором запущены Apache, Caucho, Resin и еще несколько процессов, не связанных с электронной почтой. Насколько мне известно, постфикс не установлен (which postfix ничего не возвращает).

Какие шаги я могу предпринять, чтобы правильно диагностировать и решить проблему?

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