- it's opensource
- there are many features that usually you need to pay for and plenty of additional settings
- secure - you can use own pgp for encrypted and signed messages
- for example you can disable poor ssl protocols for connections or check mx dns of email sender
- privacy - it cares about privacy (doesn't collect data, doesn't store it on some server, you don't need to grant special permissions etc.)
- there are many other privacy options eg - by default all messages are formatted in a way that prevents you from clicking in something nasty
- there is a PRO version to support creator with special features like email sync scheduler
CONS - didn't find any.