"If the guest account is enabled (on Exchange 5.5 and 2000), even if your login fails, you can send mail, because the guest account is there as a catchall," he said. "Even if you think you've done everything (to secure the server), you are still open to spammers."
The guest account is a way for administrators to let visitors use a mail server anonymously, but because of security issues, the feature is generally not enabled. Exchange servers that had been infected by the Code Red worm and subsequently cleaned will still have the guest account enabled, Greenspan said.
So you might not be done with this one after all...
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