Looks good, two very minor things. One is the indentation for the case/esac block in the start script. I guess it mixes tabs and spaces because the code doesn't line up. Not a big deal but easy to fix.

The other is the readme. Perhaps you could add a note that the two new formats were added to docker-mailserver in November 2019 and should be considered experimental until they have been tested in the wild for a while? I don't expect problems, but one never knows and then at least prospective new users have been warned.

I agree @erik-wramner, I will fix and add the comment.
Why not creating a Testing branch? Master would continue to be fed with bugfixes and minor changes when Testing would integrate big changes and enhancements. My 2 cents ;)

We could do that, but most of the work consists of fixes that everyone should get and then latest followed by stable works well. The next big thing is the upgrade to buster and there is a separate branch for that.

Grr I am fighting to correctly push my updates...

Are you done, or still fighting?

PR is good now.
Test has failed; I suspect a race condition or something like that. I cannot easily identify the root cause.
@erik-wramner can you relaunch the build?

Restarted. That error is not one of the frequent ones, but I have seen it before. Let's see what happens!

It worked, I merged the changes. Thanks!