Exchange nonexistent recipients must not be blocked.

From MS Exchange 2013 Edge Transport Server Security Technical Implementation Guide

Associated with: CCI-001308

SV-84491r1_rule Exchange nonexistent recipients must not be blocked.

Vulnerability discussion

Spam originators, in an effort to refine mailing lists, sometimes use a technique where they first create fictitious names and then monitor rejected emails for non-existent recipients. Those not rejected are deemed to exist and are used in future spam mailings. To prevent this disclosure of existing email accounts to spammers, email to nonexistent recipients must not be blocked. Instead, it is recommended that all messages be received, then evaluated and disposed of without enabling the sender to determine existent vs. nonexistent recipients.

Check content

Open the Exchange Management Shell and enter the following command: Get-RecipientFilterConfig | Select Name, RecipientValidationEnabled If the value of RecipientValidationEnabled is not set to False, this is a finding.

Fix text

Open the Exchange Management Shell and enter the following command: Set-RecipientFilterConfig -RecipientValidationEnabled $false

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