Can't send e-mail containing a specific domain in body (ilpi.com)

Hello.  My brother owns a company called ILPI his web domain is ILPI.com.  It is a legitimate company with thousands of customers.  His email address is Rob(at)ilpi.com.  Starting a couple months ago, I am no longer to Reply to his messages to my cox e-mail account.  I can send him new messages though.

Through trial and error I have determined that I can reply if I remove his original FROM address from the body of the message I am trying to send.  Further trial and error found that I cannot even send a new message if it contains "ilpi.com" anywhere in the message (except as part of his TO email address).

Same thing happens when I use for my other Cox email account.  It appears that ilpi.com is some kind of censored phrase that is not allowed in an email.  Also, any other recipients will not get the message either.  Same it I use Outlook, or Cox webmail.

I get this error message:

Message could not be sent to the following recipients: [Robert Toreki <rob@ilpi.com>] (550 - 550 <stillbill@cox.net> message rejected. Refer to Error Codes section at www.cox.com/.../email-error-codes.html for more information. AUP#CXSNDR
)