>> The enclosed message,  found in the PIKA  mailbox and shown under
>> the spool ID 8757 in the system log, has been identified as a
>> possible delivery error notice for the following  reason:
>> "Sender:", "From:" or "Reply-To:"  field pointing to the list has
>> been found in mail body.

It means (here is a boilerplate response):

                     -=-=-=-=-=-=-=-=-

Dear

If you want this distributed to all of xxxxxx-L, you will need to
remove the original "headers" that you included in your reply: the
software does not allow you to include them, thinking that it is
a "loop."

Additionally, please remove all other original signatures and text
NOT essential for context -- sometimes all you will need is a
relevant SUBJECT: RE: line in your reply.

Otherwise, simply forward to your intended correspondent.

Thanks.