Ohhkay I finally get what you're suggesting now. From something like Mastodon there's no clear way to specify.

Ehh, something to be solved but not a huge deal IMO. I think it'd have to be something custom, as there's no concept on Mastodon like Lemmy's communities, but I still stand by DNS isn't the way to solve it. Mixing it in with a hashtag might be a good way, where if you could "subscribe" to a hashtag over there, like #community@instance.tld, but then we're just talking about syntax. I actually do think there needs to be a standardization on "groups" then across the fediverse, and since Lemmy is the only one I've seen with a group syntax, I'd just suggest we standardize !