Here's some boilerplate that I send subscribers when they encounter this
situation:

Dear

If you want this distributed to all of xxxxxx, 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.