How to stop teams chat messages to old tenant?

I thought I have finished all migrations - but:

Existing chats are still available on the old tenant and every participant of that chat is answering to the old tenant.

In source tenant accounts had the UPN "".

I switched the external domain "" from source to target tenant. All source accounts became

In the target tenant they did get a new UPN" aand as an alias the former UPN:

NEW chats to both adsresses are going to the right tenant, the new one. But if users simply do answer in old chats these messages are going to the wrong user:

External participants do not even know they are talking to a dead teams chat.

How can I stop this?

Thank you in advance - Chris