Thanks for the post but this is a no go for me, will not recommend this to friends. From their FAQ :
Sunbird is not open source. Some of the messaging community believe that open source is more secure, it is our view that it is not. The more visibility there is into the infrastructure and code, the easier it is to penetrate it.
We are currently undergoing a third party audit (not Danny’s other company), that will validate our security, encryption and data policies.
Try : https://neocities.org/