There is one major problem with the implementation that I hope you can understand with an example. Suppose there are three forums - motorsports@example1.com, motorsports@example2.net and motorsports@example3.org, which eventually start mirroring each other by default. Let's also suppose that a user is, for whatever reason, banned from example1.com but not from example2.net or example3.org. Should the user try to subscribe to motorsports@example2.net, must the latter honor the ban list from example1.com and ban the user as well, or should each instance have its own ban list, knowing well that users can evade bans by subscribing to another of the mirrored communities?