The Return-Path (also called the envelope sender or MAIL FROM address) is set at the SMTP protocol level and is usually invisible to the recipient in their mail client. It's where non-delivery reports and some bounce/complaint feedback route to.
SPF checks this address, not the visible From header, which is a common source of confusion when someone assumes SPF is checking what a recipient actually sees. This is also why DMARC alignment for SPF is evaluated against the Return-Path's domain.
PulsePigeon uses a dedicated Return-Path per sending domain so bounce classification stays isolated per customer rather than shared across a pool.