Microsoft brings a long awaited feature to Exchange Online to share a SMTP domain between Microsoft 365 tenants based on the Microsoft 365 roadmap feature ID 67161. Cross-tenant SMTP domain sharing is planned to go in private preview by November 2022. That means that a public preview and/or general availability will take some extra time.
The feature is long awaited because this was or is possible with on-premises Exchange Server and some times required by organizations with several subsidiaries which have to be splitted, i.e. which have different infrastructures/Microsoft 365 tenants for certain reasons, e.g. legal or other. This can also be of interest as written in the roadmap item in case companies are in the process of mergers, acquisitions and divestitures.
At the time of writing this, there is no documentation for the private preview feature available. But I’d guess that it works similar to register a new domain for a Microsoft 365 tenant, but we’ll have to wait for more information.
Conclusion, opinion and summary
It’s great news to see that cross-tenant SMTP domain sharing is on the roadmap because it’s really often requested and needed. Especially in case of mergers, acquisitions and divestitures where you might have to plan for a Microsoft 365 tenant to tenant migration (T2T migration), this can make T2T migrations smoother, in my opinion, especially if the migration is planned for a longer period of time.
erik365.blog 
[…] Microsoft Exchange Online cross-tenant SMTP domain sharing private preview. Although I myself am a little doubtful that this thing will be available before next summer, I am already looking forward to it. According to the roadmap entry, the public preview will be available at the end of this year. Why is this important? One domain in several tenants. In some places, these are the requirements of the legal department in the company. Or there may be such a need due to a merger, acquisition or divestiture. […]
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