Exchange Outbound Connection Limit per Domain Count must be controlled.

Overview

Finding ID	Version	Rule ID	IA Controls	Severity
V-221221	EX16-ED-000200	SV-221221r961155_rule	CCI-001095	medium
Description
Email system availability depends in part on best practice strategies for setting tuning configurations. This configuration controls the maximum number of simultaneous outbound connections from a domain and works in conjunction with the Maximum Outbound Connections Count setting as a delivery tuning mechanism. If the limit is too low, connections may be dropped. If the limit is too high, some domains may use a disproportionate resource share, denying access to other domains. Appropriate tuning reduces the risk of data delay or loss. By default, a limit of 20 simultaneous outbound connections from a domain should be sufficient. The value may be adjusted if justified by local site conditions.
STIG			Date
Microsoft Exchange 2016 Edge Transport Server Security Technical Implementation Guide			2024-12-06

Details

Check Text (C-221221r961155_chk)

Review the Email Domain Security Plan (EDSP). Determine the value for Maximum Domain Connections. Open the Exchange Management Shell and enter the following command: Get-TransportService | Select Name, Identity, MaxPerDomainOutboundConnections If the value of "MaxPerDomainOutboundConnections" is not set to "20", this is a finding. or If the value of "MaxPerDomainOutboundConnections" is set to a value other than "20" and has signoff and risk acceptance in the EDSP, this is not a finding.

Fix Text (F-22925r411790_fix)

Update the EDSP to reflect the value for Maximum Domain Connections. Open the Exchange Management Shell and enter the following command: Set-TransportService -Identity <'IdentityName'> -MaxPerDomainOutboundConnections 20 Note: The <IdentityName> value must be in single quotes. or The value as identified by the EDSP that has obtained a signoff with risk acceptance.