Yes, @slave2anubis, I, and a couple others here have said that the current development build for ubuntu_bionic works. However, in order to make an “official” release, my impression is that there has to be more extensive testing and the maintainer has to be more confident that it works and is tested to make the Ubuntu bionic development build an official release.
However, for reasonable purposes (i.e., non-business critical assets that can survive being corrupted), it is reasonable to try out the Ubuntu bionic release.
As I have said a few times on here, here is the command for getting the development build on your Ubuntu 18.04 box:
$ git clone --single-branch -b ubuntu_bionic --depth=1 https://github.com/mail-in-a-box/mailinabox.git
$ cd mailinabox
# bash setup/start.sh