Mailer Daemon May Signal Your Computer's Been Hijacked as Spam Zombie:

Steps for Protecting Your Business

Unusually large amounts of Mailer Daemon notices clogging your email box can be your first signal that a spammer is using your business mail address to do his/her dirty work.

Keeping an eye on the mail that is returned to your email address is critical to protecting your good business name. This is sometimes the only way you will know that your business address is posted as the return (senders) address on thousands of pieces of mail sent out on a daily basis. Worse, you can find that your address is pasted on porn, or illegal porn messages. Next, you find that you cannot deliver your legitimate mail to your hard won list, because your email address has been black-balled by mail providers all over the internet.

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