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Lemmy interoperability with Mastodon
(feddit.ch)
A community to talk about the Fediverse and all it's related services using ActivityPub (Mastodon, Lemmy, KBin, etc).
If you wanted to get help with moderating your own community then head over to !moderators@lemmy.world!
Learn more at these websites: Join The Fediverse Wiki, Fediverse.info, Wikipedia Page, The Federation Info (Stats), FediDB (Stats), Sub Rehab (Reddit Migration), Search Lemmy
Regarding 3: you can. On Mastodon, simply meantion the lemmy Community in your Post and it will be posted to that community
I already have seen some of those and sure it works...sometimes.
If it doesnt work it's a federation issue