This feature is currently on hold and not part of our current roadmap. We do not plan to work on this idea in the foreseeable future. But that does not mean, that this can not change soon.
Currently, our focus is on improvements and exploration of new features for the website builder. Another reason for this is, actually, that this would require a complete reshape of our mail infrastructure.
With the focus being somewhere else, we also made it possible to use Sitejet with cPanel, as well as Plesk. There, you would definitely have the possibility to run your own mail server and whitelabel it as you wish.
Feel free to still upvote for this feature to be realized in the future.