Generally, the cause is using a "from" address that isn't one on your hosting account, or sending an email that unfortunately triggers the spam protection system due to its appearance. Other causes do include using a nulled or warez release of the software however, as the modifications made by the release groups can sometimes cause the software to behave erratically.
While I can't speak if the exact release you had was one of these, we were able to confirm the software installed is not a legitimate version of XenForo which may very well be why the emails aren't processing correctly. If you feel this is an error, please open a suspension dispute via your account panel.