Office 365 multi tenant architecture and deployment

Occasional Contributor

Hi community,


is it posible to connect two O365 tenents? The reason is: I have 1 company with 2 different O365 tenants. Each tenant represent a independent brunch. Merging of this tenants in one is not wished. See Attachment


Let's say we have tenant A and tenant B. Tenant B is like the holding company and is the owner of the root mail domain Users from tenant A need to be able to send and recieve messages over this tentant in their own tenant A mailbox. All the users from tenant A their default mailadress is from tenant B. 


Next feature:

Tenant B has a sharepoint site for the whole company. I will be used as intranet site. Users from tenant A would like to access the site to work together with their colleagues. The same in the other way. So if tenant A use a Sharepointsite they want to share it with tenant B.


Both tenant are cloud only. And their are no plans to place an on premise Dirsync or etc. device.

10 Replies

I think best solution is to do the merge of the two tenants. By the way, my two cents here:
(1) I don't think you can achieve the e-mail integration you want, but I prefere some of the Exchange experts guys tell your options here ( @Vasil Michev @Tony Redmond).

(2) You could potentially grant access to SPO to all the users on the other tenant using Azure B2B.

The problem here is that you cannot have the same domain added in more than one O365 tenant. What you can do is use different domains or add subdomains, and use say in's organization.


Other than that, you can forward the messages based on the default domain (for the receive part) and "spoof" the domain for the sending part. Which is far from ideal and will likely cause you troubles.


For the SPO part B2B/external users should work, as Juan mentioned.

I don't need to add the same domain to both tenants. Only sending and recieving mails over the same domain is my goal. At tenant A i can work maybe with a exchange sending connector. But what a bout tenant B? He need to redirect incomming mails for users in tenant A to Tenant A?


Is maybe a better solution when i install one AD Server in Azure and connect both clients to them?

You need it as accepted domain in order to receive messages. As I mentioned there are several workarounds (use the default domain, use subdomain, use forwarding), but it depends on the details.


Not sure what a server in Azure will help you with the task at hand. Is the question perhaps about only a few people needing such functionality, so that you can for example configure both tenant-a and tenant-b profile in the same Outlook client?

Not really, because tenant A don't have their own external mailadress.  Only using the mailbox in tenant A but sening and recieving mails should go over tenant b.


My Idear was to place a Ad Server in Azure with AD Synch and connect both tenants to them.


Sending messages from Tenant B to Tenant A is easy. I create a contact in tenant b with a riderect to the onmicrosoft address of tenant a. You need to set the smtp alias from tenant b as default in this contect after creating. Now when i send a mail to it will be redirectet to In the mail the recieptentadress is So incoming is working good.  Know i need tell tenant-a that they can send in the name of But i can't add an accepted domian to the exchange of tenant-a.


How can i configure the exchnage on tenant a to connect to tenant b for sending mails in the name of tenant b? 

Hi Christoph


As @Vasil Michev mentioned, it's not possible. A user in Tenant A who has smtp addresses from domain cannot have an smtp address from to send emails. Reason is is not added to Tenant A and the domain is already added to Tenant B. Therefore you cannot add that doamin to Tenant A or vice versa.


Best option I see here is, if the user count that requres this ability in Tenant A is less, create accounts for those users in Tenant B.


Thank you.

Muditha Chathuranga

For the Sharepoint requirement, have you looked at using Azure AD B2B?




@Muditha Chathuranga and @Vasil Michev i have got an other idea. Is it possible to share one mail domain with 2 O365 tenants over an hybrid configuration?


So what i mean is, to use one exchnage 2016 onpremise and connect them to 2 seperated O365 tenants. Than use the onpremise Exchange as mailrouter to share one mail domain in 2 tenants. What do you think?

@Christoph Wolf @Juan Carlos González Martín 


I am interested on how you got on with this requirement given the latest innovations.  Indeed I believe there is concept of "tenant friending" but I am not sure how this working practice.  


I too have the tenant A and tenant B .   The ideal is the 

  • To publish news posts in tenant A and have this surface in tenant B ( everyone in tenant A, everyone in tenant B) .  Then there is page / news templates and org level assets ( would this needed to duplicated.
  • To be able to publish and share SharePoint PowerApps and Flows across both tenants
  • To roll identical or complimentary governance across both tenants
  • Handle search, term ids, content types etc in an elegant way  - hybrid v2 :beaming_face_with_smiling_eyes:

Are there workarounds for any of this I wonder ?