Not quite. So imagine each Lemmy instance is a post office, responsible for receiving mail from other instances on your behalf, and also responsible for sending your mail to other instances.

The post office (instance) that matters is the one where the thread is taking place. When you write a message on a thread - if the thread is on your home instance, great! No outbound mail. If it’s not, your post office (instance) will send your mail across the fediverse to the instance where the thread actually lives, and the receiving instance adds it to the thread.

Likewise, when users reply to you, their instances mail their replies to the instance where the thread lives; the thread’s instance is then responsible for re-forwarding their mail to you.

In this case, Beehaw has unilaterally decided not to accept mail from, or deliver mail to, Lemmyworld. So your experience varies based on which is your home instance, and which instance you’re interacting with.

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So exploring it by example, let’s consider we have three instances: Beehaw, Lemmy.world, and (shameless plug) my adorable little home instance of federate.cc, which is federated with both of the above (and is accepting membership applications by the way!)

If the thread lives on Beehaw, and you are a member of…

• Beehaw, then you can interact with the post (it’s your home instance), but you won’t see any replies from Lemmyworld users (your post office is refusing to accept letters from them)
• Lemmyworld, you probably won’t be able to see the thread at all (Beehaw refused to deliver it to their post office) - but if by some chance you can, you won’t see any comments by Beehaw users (Beehaw refuses to deliver them to the Lemmyworld post office)
• federate.cc, then you will be able to interact with the post (Beehaw delivers mail to our post office), but you won’t see any replies from Lemmyworld users (Beehaw refuses to accept mail from Lemmyworld users, and by extension refuses to deliver their mail to our post office)

If the thread lives on Lemmyworld, and you are a member of…

• Beehaw, you probably won’t be able to see the thread at all (Beehaw refused to accept the mail from Lemmyworld, returning it to sender) - but if by some chance you can, you won’t see any comments by Lemmyworld users (Beehaw refuses to accept mail from Lemmyworld, returning it to sender)
• Lemmyworld, then you can interact with the post (it’s your home instance), but you won’t see any replies from Beehaw users (their post office refuses to deliver letters to yours)
• federate.cc, then you will be able to interact with the post, but you probably won’t see any replies from Beehaw users (Beehaw’s post office refuses to deliver mail addressed to Lemmyworld)

If the thread lives on federate.cc, and you are a member of…

• Beehaw, you will be able to see and interact with the thread, since Beehaw’s post office both sends and receives mail from federate.cc; but you won’t see any messages from Lemmyworld users, because Beehaw’s post office will refuse mail that originated at Lemmyworld; when we try to forward that mail to Beehaw’s post office, it will be refused and returned to sender.
• Lemmyworld, you will be able to see and interact with the thread, since Lemmyworld’s post office both sends and receives mail from federate.cc; you will see messages from Beehaw users, because Beehaw’s post office will deliver mail to ours; but if you interact with them at all, they will never see it, because Beehaw will reject mail that originated at Lemmyworld (see above).
• federate.cc, then you will be able to interact with the post as normal, you will see replies from people at all three instances, and they will see yours

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Phew! How confusing. A far better solution would be… don’t defederate, splitting the community!