Using Synaptic (or CLI, doesn’t matter), remove all of the packages with “evolution” in their name, with the exception of:
You may also need to enter
- sudo rm /usr/share/indicators/messages/applications/evolution
Installing Thunderbird in the usual manner should go smoothly after that. I don’t remember if it asks to be the default, but if it doesn’t, just go to Preferred Applications and make the necessary change.