I ran into this recently even on Facebook Messenger. A friend of mine was hunting for a short domain name and I had a list of some three character .net and .org domains I recently had found that were available.

Cut and pasted the list and the message wouldn't send.

Narrowed it down to one. Typed just the bare domain. Wouldn't go through. (It was something incredibly benign like n17.org)

Couldn't find a history on that domain name for why it would have been filtered.

At least messenger responded with 'couldn't send message' but still no clue as to why... and it took me sending each domain name individually until I found the one that was failing the entire message.