Generate Postfix virtual_alias_maps file
In my setup I've Postfix and Mailman3 running. Postfix uses configuration data from a MySQL database which allows catch-all aliases for domains using Postfix virtual_alias_maps option.
Due to the way Postfix evaluates its configuration the catch-all alias from the database always takes precedence over all config files Mailman3 generated for Postfix. The only workaround I was able to figure out is injecting all mail addresses Mailman3 should handle in the aliases configuration with an alias to itself, e.g aliasing "email@example.com" to "firstname.lastname@example.org". This workaround works because Postfix uses the most specific alias definition during this part of the config evaluation.
The workaround is OK for me at the moment as I'm still testing Mailman3 and the its Postfix integration, but I would try to avoid this additional configuration for each mailing list after the test phase. My goal would be to use Mailman3 tools to configure mailing lists and the database with its interfaces to configure everything around standard mailboxes.
If Mailman3 would write an additional Postfix map file, which maps each mailing list alias to itself, this map file could be used as an addition for the virtual_alias_maps configuration option in configurations like the one above.
For my running debian version from stretch-backports (3.1.1-9~bpo9+1), I already modified the code locally and everything seems to work fine. Within the next days I'll port my changes to the current master and link this issue to a merge-request.