Delivery has failed to these recipients or groups:
This message is larger than the size limit for messages. Please make it smaller and try sending it again.
Diagnostic information for administrators:
Generating server: MAIL.CUSTOMER.COM
#550 5.3.4 ROUTING.SizeLimit; message size exceeds fixed maximum size for route ##
Customer sending an email (with an attachment) to us.
Based on this bounce message only, which side is at fault here?