@LincDavis most likely is on the right path, this being a spam related issue, but the 554 error message you received can have many meanings. As you can see on this page, the 554 error can be associated with numerous types of issues:
554 Invalid recipient (BAD EMAIL ADDRESS)
554 rejected due to spam content (SPAM RELATED ISSUE)
554 Malformed mail denied!
I know this thread is a little over a week old, but I wanted to touch base and find out if you ever found a solution? I'd like to update my documentation on 554 errors, and knowing the issue you had behind "554 Denied" will be helpful.