ActivityPub addressing is really somewhat complex and pretty much any server is going to abstract it with things like at-mentions, sometimes making different assumptions. When I reply to lemmy posts from my microblog.pub server, I sometimes need to make sure to include both the user (@[email protected]) and community (@[email protected]) if they happen to be on different domains. Otherwise my server’s addressing won’t send it to the right place. And I doubt other lemmy instances will see this post unless I tag them too.
@[email protected] @[email protected] Sure, although I had forgotten that the community shares replies to, so it actually does federate out if I tag it.