The 554 mail error is a catch-all category for a wide range of email failure issues. What's most important is the entire body of the bounce message containing the error, which will generally help point in the right direction. There isn't a single solution to 554 errors. This depends on the more specific cause detailed in the bounce message itself.
If any confusion on how to proceed, please submit a ticket from your client area, including the bounce message, so that we can help troubleshoot and resolve the particular cause.