Firewall security
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Ensure that inbound connections on port 25 are only accepted from our IPs. The full list list of ranges can be found here: http://images.messagelabs.com/EmailResources/ImplementationGuides/Subnet_IP.pdf
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Password security
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Ensure that you do not have any test accounts or demonstration accounts with generic or easily guessed passwords.
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Guest accounts
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Disable guest accounts. (Guest accounts usually have the user name "Guest" and a default password. If you disable the guest accounts, you reduce the risk of an attacker breaking into the system.)
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Open relaying in Microsoft Exchange
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Spamhaus' recommendations
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Check http://www.check.spamhaus.org/ for spam control recommendations.
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If your IP has been removed for outbound spam relay, before contacting Support to have it re-enabled, please consider the following:
Mail server log and firewall log
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Queued spam
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Delete any spam from the outbound email queue on your server to prevent your system from being blacklisted once the service is reconnected.
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