What is a return path domain?

The return path domain is the domain used in the Envelope From address. It receives bounces and is evaluated during SPF checks and DMARC alignment. It is the manifest address where any undeliverable cargo is returned.

This domain is also known as the Envelope From domain, the Bounce Address, or the HELO identity. In most ESP delivered mail it differs from the visible sending domain which is exactly why DMARC alignment exists.