Shared mailbox access from another O365 tenant

[gmtsolutions]

We have a client with two separate companies - call them AB and ABCD.  They were originally set up under one Office 365 tenant, using two different domains, but were then split as they both needed the same mailbox addresses (sales@ab.com and sales@abcd.com for example), which was not possible under the same tenant.

They now each have an Office 365 setup with their respective domains.  However we now have a situation where users from company AB need to access the shared mailbox on ABCD (sales@abcd.com) which is in a different Office 365 tenant.  Is there any way to set this up?

[#2]

Hello you cannot assign a permission to a shared mailbox to another user from another tenant.

Regards.
Alejandro.

[#3]

@gmtsolutions Too bad it wasn't a custom domain ;) Shared mailboxes are useful and suit their purpose (we have a bunch of them). But would it be an alternative to set up M365 groups instead as your current need isn't supported?

[#4]

you can create a distribution group to send a mail to another address, but you cant reply to from the domain of original tenant. In your original question, you said you need to split in two tenants because you cant get the same address sales@domain1 and sales@domain2 but this it´s posible. For me the best is use only 1 tenant with two domains, using some users with domain1 and others with domain2, and you can use shared mailbox with differents domains or not and you can set access between all users using any domain, but all of this with only one tenant.

Let me know any question.
Regards
Alejandro.

[#5]

Mailboxes can only be accessed by users in the same O365 tenant, what you are asking for is not possible.