When you create or update an Exchange hybrid configuration using the Hybrid Configuration Wizard magic things happen. That’s why it is called a Wizard.
One essential step of the Hybrid Configuration Wizard (HCW) is the configuration of the hybrid mail flow. The hybrid mail-flow is required for both, classic and modern Exchange hybrid.
The wizard asks you to select one or more Exchange servers that you will utilize for handling inbound mail traffic from Exchange Online to your on-premises organization. You either configure direct mail flow to your Exchange Mailbox Servers in your internal company network, or to your Edge Transport Servers located in the perimeter network.
The following screenshot example shows the selection dialogue.
You can only select a server object, but not a receive connector on that selected server. The HCW chooses the “right” receive connector on the selected servers for you. If you are using the default set of receive connectors, you will not encounter any issues. HCW will use the default frontend connector on a mailbox server. When you use an Edge Transport Server you will run into any trouble as well. There is only one receive connector which you must extend by setting some additional parameters.
But what about an Exchange Organization where each mailbox server hosts multiple receive connectors bound to TCP port 25?
The Problem
When you use multiple receive connectors bound to TCP 25 you will see that HCW will choose a receive connector that you won’t expect. You might think that HCW will select always the default frontend connector. That is not the case.
When you select multiple servers for hybrid mail-flow, and each server has a different receive connector configuration, you might get the impression that HCW selects the receive connector randomly. That is not the case either.
While doing some testing in a large enterprise infrastructure with five different Exchange forests (development, testing, staging, pre-production, and production) we saw an interesting behavior.
From all available receive connectors having a TCP 25 binding, HCW selects the receive connector with matching RemoteIPRanges values of:
- IPv6 all (::-ffff:ffff:ffff:ffff:ffff:ffff:ffff:ffff) and IPv4 all (0.0.0.0-255.255.255.255)
This is normally the default frontend receive connector when you do not adjust the RemoteIPRanges parameter - Just IPv4 all (0.0.0.0-255.255.255.255)
- Just IPv6 all (::-ffff:ffff:ffff:ffff:ffff:ffff:ffff:ffff)
- IPv6 any address and IPv4 any address
- Just an IPv4 address
Adjusting the default receive connector does have a direct impact on how HCW selects a receive connector in your Exchange environment. When you use multiple receive connectors for internal relay purposes, your receive connectors might end up in a messing situation. As mentioned, HCW selects receive connectors with a TCP 25 binding, regardless of the transport location of the connectors, frontend, or hub transport. The enterprise environment mentioned had some deviations between the different environments and we saw TCP 25 receive connectors in frontend transport and hub transport.
The Solution (sort of)
Run the HCW and select only one server for hybrid mail-flow and identify the receive connector configured by HCW. Configure an appropriate receive connector on all other mailbox servers used for hybrid mail flow. Update the hybrid configuration object of your on-premises Exchange Organization accordingly.
Verify the following two Tls* parameters of the receive connector:
Get-ReceiveConnector 'EXSRV01\Default Frontend EXSRV01' | fl tls*
TlsCertificateName : CN=Sectigo RSA Domain Validation Secure Server CA, O=Sectigo Limited, L=Salford, S=Greater Manchester, C=GBCN=mail.varunagroup.de, OU=PositiveSSL, OU=Domain Control Validated
TlsDomainCapabilities : {mail.protection.outlook.com:AcceptCloudServicesMail}
You must ensure that the hybrid receive connector uses the correct TLS certificate, enabled for SMTP. Additionally, you must set the TlsDomainCapabilitiers to allow cloud mail for connections incoming connections with a TLS certificate for mail.protection.outlook.com.
Keep your receive connectors at frontend transport.
Links
- Exchange Server Mail Flow and the Transport Pipeline
- Exchange Hybrid – What, Why, and How – Presentation @ Slideshare
Enjoy Exchange Server.
