Thank you for your email. This error is not a SafetyCloud error, but rather is an error being sent back to you when delivery of the message fails. The link that you are given next to the error code tells you the source of the issue, and this is not something that we can control, nor do we have any influence over it. To resolve this issue, you will need to troubleshoot the root cause (in the link:
http://www.mimecast.com/knowledgebase/KBID10473.htm#554) with the intended recipient of the email or their mail administrator.
If there is anything else that I can assist with, please don't hesitate to let me know.