If you write [email protected] and aren't on some.instance yourself, that works for everyone, regardless of whether their instance server knows about that community already or not.
If you are on some.instance and write [email protected], both lemmy and kbin are overzealous and it ends up only working for people on some.instance.
In that case, you can use [link text for lemmy users](/c/[email protected]) and [link for kbin users](/[email protected]). The one for kbin users looks a bit different to guarantee the link works even if their instance doesn't have anyone subscribed to [email protected] yet. Not sure if the lemmy link works in such a case.````````