Project membership information can be pulled from the OpenStack keystone API.
The Toolforge project itself [[https://wikitech.wikimedia.org/wiki/Help:Toolforge#Mail_to_a_Tool|has something like this]] that routes emails for `<tool>@tools.wmflabs.org` to the shared tool account where it can be processed using `.forward` files. That processing is LDAP based, using a [[https://phabricator.wikimedia.org/source/operations-puppet/browse/production/modules/toollabs/files/maintainers|lookup script]] and [[https://phabricator.wikimedia.org/source/operations-puppet/browse/production/modules/toollabs/templates/mail-relay.exim4.conf.erb|exim4 configuration]].
Possible implementations:
* `*@<project>.wmflabs.org` forwards to project admins
* `<project>@<something>.wmflabs.org` forwards to project admins
* `<project>-(admins|members)@<something>.wmflabs.org` forwards to project admins/members
* ...
Having some kind of optional per-project address -> destination mapping similar to the #toolforge feature would also be nice, but that is not necessary for an initial implementation.
Ideally this base feature can also be leveraged to fix {T61142} either directly or indirectly.