I tried a completely clean install and narrowed down what happened.
We changed recipient_delimiter in /etc/postfix/main.cf from + to - (because that’s what was on the old mail server). This worked for earlier versions of mailinabox.
After a recent upgrade it stopped working and even changing back to + for testing did not work.
Changing to both ± (which is valid on the version of postfix mailinabox uses) doesn’t work either.
So there must have been a recent update to that file by an MiaB update. The script does not replace files and instead kindly inserts a # in front of lines that should not be there.
You will have to check this line after every update of MiaB.
I’d been migrating a second domain from my old server to box, so it’s possible I’d switched some users to accounts rather than aliases, which could be why it stopped working. I didn’t log every change though, so that’s speculation.
I have it scripted now, so I can patch easily after an update.