Could just be a spam filter reaction to a lengthy code dump, etc.
See: For Best Results - Read the Label FAQ 1, etc., and how to paste code, if that's what you're attempting...use the <> brackets in the editor.
Still no joy, paste it all into a new email, via 'paste and match style', then select all/copy, then create a new topic here and do another 'paste and match style.
Still no still no joy, strip your post down until it's accepted, then add blocks of content until it breaks, indicating the offending invalid content.
Do not attempt to add images, videos, etc.
I have the same issue. Only text and nothing fancy at all. Been searching for it here and seems like there could be banned words or something as well. Seems to have been likte this for ages... But to not explicitly tell the user what characters that's invalid is just bad.
...edit, played with it more now and apparently the word r e m a r r i a g e is banned. So freaking odd.