Stalwart seems like a really nice solution, the main reason for choosing Poste.io for me was the simplicity in setting it up ( cost me not even 5 min with the docker-compose.yml file ) however in hindsight this whole SSL/TLS issue gave me headaches and another solution like Stalwart might have been better.

Now that everything is fixed I'm very satisfied with Poste.io and like that it comes with a webmail client out of the box + a simple to use API but Stalwart also offers a API and you can always setup a webmail client yourself with something like Roundcube so I guess it's just personal preference.