Oh wow I had no idea these logs existed. Great catch. We should add something to the setup to take care of this. Looks like /etc/systemd/journald.conf contains settings for this, currently:
#SystemMaxUse=
#MaxRetentionSec=
which according to the documentation are the max total disk size and the max retention length (time). I’m not sure what is preventing the logs from staying around forever because Ubuntu has these defaulted to off, but in any case, we could try setting the retention length (MaxRetentionSec) to 10 days, as you tried, during Mail-in-a-Box’s setup. I think that’s a reasonable default.
What do you think?