Company aquisition with another 365 tenant

[jsouter]

Hello folks

Our business >1400 employees has bought out another business >500 employees as we are the group business we will want to onboard these users and data Sharepoint/Teams/OneDrive and Exchange mail + archives at somepoint this coming year to our tenant.

I have some questions around this:

1, What are the best practices for merging these 2 tenants, are there any guides or 3rd party SW that can assist with moves?

2, Some of the other businesses users have a need to access a shared mailbox on our current tenant, is this possible without having to create them a new user account on the main tenant as we are already paying for Lic's on the other side? can external guest access be somehow used?

Thanks for any input

[#2 · Accepted solution]

Hi @jsouter,

For Cross tenant mailbox migration I always use the MS solution: 

Cross-tenant mailbox migration - Microsoft 365 Enterprise | Microsoft Learn

I know that there're a lot of tools out there that can do that, but this process is quite easy and I performed many migrations without issues.

For Teams, ODB, SPO... The tool that I tested more and know that works quite good is ShareGate:

SharePoint migration tool - ShareGate

Again, there're much more ones out there, but I have no personal experience with those ones.

Regarding your second question... Unfortunately that's not possible. An alternative could be create a Microsoft 365 Group. You can find more info here: How to give access to a shared mailbox for a Guest/External user? - Microsoft Q&A