Friday, January 4, 2019

Exchange Multi-Forest Hybrid Tips and Tricks

Setting up Exchange hybrid between Office 365 and on-premises is a common and well-understood configuration.

Multiple forests, each with their own hybrid connection into a single Office 365 tenant, is supported from Exchange 2010 to 2019, although it does come with a unique set of challenges. This article assumes you have one Exchange organisation in hybrid and multiple Active Directory Domains synchronizing using Azure AD Connect already and are looking to add more Exchange Forests.

Office 365 tenant with two hybrid email forests

Key Considerations with Multi-Forest Exchange Hybrid

The basics still need to be in place for additional Exchange hybrid deployments. Azure Active Directory (AD) needs to successfully sync all users to the cloud (one per tenant, regardless of how many AD forests), Sender Policy Framework (SPF) and Domain-based Message Authentication. Reporting & Conformance (DMARC) also needs to be configured correctly, and the Simple Mail Transfer Protocol (SMTP), Embedded Web Server (EWS), and Autodiscover on-premises endpoints need to be exposed properly.

We should also take care to ensure that all Autodiscover endpoints work for all – internal name resolution allows connections through internal firewalls and if Service (SRV) records are in use then Outlook Autodiscover prompts are suppressed.

Hybrid Configuration Wizard
Setting subsequent Exchange organisations into Federation can be achieved by the normal Hybrid Configuration Wizard (HCW)

Centralised Mail Routing
Centralised mail routing is Microsoft’s term for sending outgoing emails via your on-prem environment. If this is present, or you wish your newly joined Exchange forest to perform centralised mail routing, then it’s necessary to take care to avoid unexpected mail routing. centralised mail routing and deliver email straight from Office 365 via the implicit deliver-via-MX-resolution route.

Don't enable centralised mail

Organization Configuration Transfer
In the Hybrid Configuration wizard, there is an option to perform an ‘Organization Configuration Transfer’ which copies certain configurations like retention policies tags.
Consider carefully before enabling organisation configuration transfer
Routing address
As part of the Hybrid Configuration wizard, your Exchange Address Policy generation default rule will be updated to include an Office365 routing address:

Use Email address from Company A in Company B
Once Exchange organisations are hybrid-enabled into the same tenant, it is possible for a user to have an email address based from another forest in the environment.

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