Microsoft Teams can't rejoin a Guest Forum

[Issen007]

Hello,
We have a new sister company that we invite to one of our Microsoft Teams channels.

So when our holding company bought them, we invited them as a guest to our tenant and everything worked well.
Now that they have moved their Office 365 accounts from their old domain to our domain structure as a tenant, they could no longer join the Teams channel.
So we remove everyone from our Microsoft Teams Channel and all "Guest Accounts" in Azure Active Directory.
Waited 72 hours and then invited them back to our Teams Channel.

Now for some reason they end up in a loop where they agree to join our channel and get the welcome message, Teams then try to log in to our domain but there it just hangs until it finally makes a timeout and came back to the first screen there they must agree to join our Teams Channel.

So my question, are there more places than Team & Azure AD where there are traces of old accounts?

Thanks

[#2]

Hello, I'm trying to understand the scenario you are describing. What is the setup more precisely here? Custom domains under one tenant? A permanent deletion in AAD of the guest accounts (not only a deletion, also deletion from the bin) followed by 24 hours of waiting should be enough to remove them.

[#3]

@ChristianBergstrom So the setup is farily easy.

They had a shared M365 tenant setup before that was join our Teams Channel.

They have now migrated (Export/Import) does e-Mail address from that tenant to a new own tenant. 

That mean the email address was register with a old tenant ID. 

I haven't test to clear the bin, I'll will test that also.