Yes - very good!

Hello! Grammatically, the sentence is correct. I would write it a little differently just for the sake of clarity. Please be informed that I have standing instructions from my customer to forward document X to you. There is no need for explicit permission. I hope that this was helpful. Christine

Yes, this is great! If you want to emphasize that you do not have specific instructions on how to forward the document or what to do afterward, you could say "Please be informed that I have standing instructions from my customer / client to forward document X to you, however, I was not given any specific further instructions." Or if you want to emphasize that you have standing instructions, but you have not been instructed as to this specific email / document, you could say "Please be informed that I am forwarding document X to you based on standing rather than specific instructions from my client."