Microsoft Teams can’t chat with external Domains
When users in your organisation try to chat with users outside of your organisation, they receive the following message in Microsoft Teams.
“We can’t set up the conversation because your organisations are not set up to talk to each other”
To allow your users to communicate / chat with users outside of your organisation, you will have to allow external access in Microsoft Teams. To do this follow below.
Open Microsoft 365 Admin Centre > Teams then click on Org-Wide Settings > External access.
Enable “Users can communicate with Skype for Business and Teams users” as well as “Users can communicate with Skype users”
You also have the option to block or allow domains if your organisation only wants to allow external communication with certain domain.
To block communication with an external domain clock on “+Add a domain”
Type the domain name and click on “blocked” then click Done at the bottom of the page.
The domain will be listed under domains with a status of Blocked.
Once you have configured your external access your users will be able to communicate to external organisations.