Invoice out-put type issue

Dear Expert,

I am creating an invoice where I got both RD00 (Printing invoice) as well as output MAIL(out going mail).

Both picks because of condition record.

RD00 picks with key combination of Sales organization + Billing type. So it's not specific to any customer.

Now my client wants when MAIL output trigger RD00 should not consider for specific customer. And they do not want to touch any condition record.

They want to achieve through user exit. I have used Z-program and forms for above out-put.So please advise how to proceed.

Regards

Ramesh