Exchange Receive connectors must control the number of recipients per message.

From Microsoft Exchange 2016 Edge Transport Server Security Technical Implementation Guide

Associated with: CCI-001095

SV-95249r1_rule Exchange Receive connectors must control the number of recipients per message.

Vulnerability discussion

Email system availability depends in part on best practice strategies for setting tuning configurations. This configuration controls the maximum number of recipients who will receive a copy of a message at one time. This tunable value is related to throughput capacity and can enable the ability to optimize message delivery. Note: There are two types of default Receive connecters:"Client Servername" accepts SMTP connections from all non-MAPI clients, such as POP and IMAP. As POP and IMAP are not authorized for use in DoD, these should not be present. Their default value for MaxRecipientsPerMessage is 200."Default Servername" accepts connections from other mailbox servers and any Edge Transport servers. Their default value for MaxRecipientsPerMessage is 5000.

Check content

Review the Email Domain Security Plan (EDSP). Determine the Maximum Recipients per Message value. Open the Exchange Management Shell and enter the following command: Get-ReceiveConnector | Select Name, Identity, MaxRecipientsPerMessage For each receive connector, if the value of "MaxRecipientsPerMessage" is not set to "5000", this is a finding. or If the value of "MaxRecipientsPerMessage" is set to a value other than "5000" and has signoff and risk acceptance in the EDSP, this is not a finding.

Fix text

Update the EDSP to reflect the Maximum Recipients per Message value. Open the Exchange Management Shell and enter the following command: Set-ReceiveConnector -Identity <'IdentityName'> -MaxRecipientsPerMessage 5000 Note: The value must be in single quotes. or The value as identified by the EDSP that has obtained a signoff with risk acceptance. Repeat the procedure for each receive connector.

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