What is a return-path or bounce domain?

The return-path (also called bounce domain or envelope from) is the address where bounces are sent. It's specified in the SMTP envelope, separate from the visible From header. When mail can't be delivered, the receiving server sends failure notices to this address.

SPF authentication checks this domain, not the header From. For SPF to align with DMARC, the return-path domain must share the same organizational domain as the header From. This is why ESPs encourage custom bounce domains.

Without a custom return-path, bounces go to the ESP's domain, and SPF alignment fails under strict DMARC policies. Setting up a custom bounce domain typically involves adding a CNAME record that points your subdomain to the ESP's bounce handling infrastructure.

The return address on the envelope. It's where the post office sends mail that couldn't reach its destination.