Just for the records, the YNH packaging of Mailman3 was just updated to v. 3.3 instead of 1.0… but this doesn’t mean that the underlying Mailman3 installation has been upgraded. This will only happen once YNH updates to the latest Debian release. So, still stuck to not so recent version of Posterous, and their issues :-/
Just thought that this could be useful to some, saving some time wondering what will be updated exactly (and bumping into Would be nice to have a changelog / forum announcement about releases · Issue #44 · YunoHost-Apps/mailman3_ynh · GitHub )