hm, that's quite a drawback in the case of relay.mostr.pub, which is a bridge to Mastodon. it's far less effective as a bridge if nothing from there is even possible to share to people who aren't using it.
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but as i understand it, i can rebroadcast anyone's message that is visible to me to all the relays i'm connected to. would i have to do that prior to sharing so that people will see it?