In many situations or scenarios Microsoft Outlook, instead of regular (consumable by other mail clients) meeting invitations can start sending “windmail.dat” instead. This is a proprietary format that other mail clients (like Thunderbird, Mac mail clients, web clients) ignore as not readable.
List of situations and configuration switches that may cause Outlook to behave like this is long. And so is the solution list. In my case only the most aggressive solution worked.