I'm not an expert on the details of the protocol, so I can't give you a good answer to that. I believe it's just a matter of Mastodon not accepting messages tha

I'm not an expert on the details of the protocol, so I can't give you a good answer to that. I believe it's just a matter of Mastodon not accepting messages that it knows it won't be able to display correctly. But other "microblog"-oriented Fediverse instances can exchange messages with Mastodon instances just fine.