An engineer must limit responses from the gateway that are directed to invalid email addresses. How should the LDAP server be configured to accomplish this goal?

Question

Options

• AValidate the sender email address via an LDAP query during the SMTP conversation.
• BValidate the sender email address via SMTP Call-Ahead to query an external SMTP server.
• CLimit the number of invalid recipients per sender to stop responses after crossing the threshold.
• DLimit the number of invalid responses per recipient to stop responses after crossing the threshold.

• A
  8% (3)
• B
  11% (4)
• C
  78% (29)
• D
  3% (1)

Explanation

This question addresses Directory Harvest Attack (DHA) prevention. When spammers probe a mail gateway with many random recipient addresses to discover valid ones, the gateway generates many 'user unknown' responses. Limiting the number of invalid recipients per sender (option C) causes the ESA to drop or throttle the connection once the threshold is crossed, preventing further probing without requiring LDAP lookups. Option A validates senders (not recipients). Option B (SMTP Call-Ahead) validates recipients by querying a downstream SMTP server - useful but not the described approach. Option D is not a standard ESA configuration parameter. Configuring a threshold per sender is the direct ESA mechanism to stop responses to invalid addresses.

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