Hi Whitney,
Marketo receives these messages from the receiving email server. A 554 bounce is a response that is described as "The email bounced for an unspecified reason", which I know is not very helpful. Which email server you received this message from did not include any details in the bounce message.
The best thing to do to avoid this in the future would be to make sure you have DKIM, SPF, and branded links enabled to improve your overall deliverability.
Here are some articles:
DKIM/SPF: http://community.marketo.com/MarketoArticle?id=kA050000000KyrDCAS
Branded Links: http://community.marketo.com/MarketoArticle?id=kA050000000L3QKCA0
Will