If the Messaging Service stops, it is automatically restarted as long as the computer itself is otherwise healthy, the Messaging Service should restart. In this case, if the configured instance fails, Tableau Server will still work but status will show as 'Degraded', and users mentioned in comments will not receive email notification of the mention. If you have a single-node installation of Tableau Server, or are running version 2019.4.x, you are limited to a single instance of the Messaging Service.

In this case, if one of the instances fails, the other instance handles all traffic, and you can remove the failed instance and reinstall an instance. You can have two instances of the Messaging Service configured if you have version 2020.1 or later, and you have a multi-node installation of Tableau Server, you can install two instances of the Messaging Service. Multiple instances of Messaging Service (multi-node, version 2020.1 and later) The actual impact depends on whether you have one, or two instances of the Messaging Service configured. If the Messaging Service stops or fails to start the consequences are relatively mild. Impact if Messaging Service is not running properly

Messenger server status