The bounce error message is along the lines of the following where xxx.xxx.xxx.xxx is an IP address and the domain is that of the sending address.

550-[SPF] xxx.xxx.xxx.xxx is not allowed to send mail from {somedomain.com}.
550 Message blocked - Please check settings

This message means that the sender domain's DNS included an SPF record which specified which servers were permitted to send email from that domain and this message originated elsewhere.  The sender should use one of the domains which were allowed or have the SPF record corrected to reflect the correct servers.