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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-221221r612603_rule 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 2020-12-10

Details

Check Text ( C-22936r411789_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 value must be in single quotes.

or

The value as identified by the EDSP that has obtained a signoff with risk acceptance.