That message is what the recipient's mail server told your Messaging Gateway when it tried to deliver the email. I performed a search on Google, and I see that this is a response that MIMEcast is known to give when it is not correctly configured to receive mail from your IP address. Here is the thread for more information:
https://community.mimecast.com/mimecast/topics/outbound_messages_returned_550_administrative_prohibition_envelope_blocked
Basically, the recipient cannot accept your message becuase their mail server is configured to not accept mail from you. You should contact company.com and resolve this with their network administrators.