When receiving an email , one can hover on the person's picture or name to display a contact card.
On the contact card there are icons to Send an email and also to start a Chat (IM).
For internal contacts (employee of our own Enterprise and working on same tenant) the Chat icon is active (blue) and opens a Teams window to start chatting. For external contacts (employees from Microsoft, MSFT Partners, or other vendors) the Chat icon is not active (grey), even though we can start a Chat with these people directly in Teams. Why is it not working from Outlook Web? In Outlook Desktop this is working as expected. Any idea what our configuration is missing? In Teams admin we selected the Open Federation option. See attachments.