Exchange nonexistent recipients must not be blocked.

Severity	Group ID	Group Title	Version	Rule ID	Date	STIG Version
medium	V-259613	SRG-APP-000261	EX19-ED-000127	SV-259613r961161_rule	2024-12-06	2

Description
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 nonexistent 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 versus nonexistent recipients.

Description

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 nonexistent 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 versus nonexistent recipients.

ℹ️ Check
Note: If third-party anti-spam product is being used, the anti-spam product must be configured to meet the requirement. Additionally, the default value for "RecipientValidationEnabled" is "False". Open the Exchange Management Shell and enter the following command: Get-RecipientFilterConfig \| Select-Object -Property Name, RecipientValidationEnabled If the value of "RecipientValidationEnabled" is not set to "False", this is a finding.

ℹ️ Check

Note: If third-party anti-spam product is being used, the anti-spam product must be configured to meet the requirement. Additionally, the default value for "RecipientValidationEnabled" is "False". Open the Exchange Management Shell and enter the following command: Get-RecipientFilterConfig | Select-Object -Property Name, RecipientValidationEnabled If the value of "RecipientValidationEnabled" is not set to "False", this is a finding.

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