I've never heard of the Bounces-To header, and googling for it reveals precious little. I see that Amazon uses it, but there doesn't seem to be an RFC which mentions it. There isn't even a wikipedia article about it, and it is not mentioned in their article on Bounce message. Are you sure that it is widely supported?

The way I know to handle this is to set the Return-Path to your bounce address, and the From header to the value that you want the user to see. For instance:

   Return-Path: user_123@bounces.domain.com
   From: help@domain.com
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Update: Of course, you would also need to process the bounce messages correctly, distinguishing temporary from permanent failures. There is the module Mail::DeliveryStatus::BounceParser but I've never used it and have no idea how effective it is.