What is a “Return-Path”?

The Return-Path header (also called bounce address or envelope from) specifies where delivery failure notifications should go. It's set from the SMTP MAIL FROM command and recorded by the receiving server.

Example: Return-Path: <bounce-abc123@mail.yourdomain.com>

Key points:

Bounce handling: When delivery fails, the error message goes to the Return-Path address, not the From header. This lets senders track bounces through dedicated mailboxes or automated processing.

SPF authentication: SPF checks the domain in Return-Path, not the visible From. This is why custom bounce domains matter for alignment.

Often different from From: ESPs use unique Return-Path addresses per message or campaign. This enables bounce tracking and identification. The recipient sees your brand in From; bounces route to the ESP's processing system.

DMARC alignment: For SPF to contribute to DMARC pass, the Return-Path domain must align with the From domain (same organizational domain).

When troubleshooting authentication, always check Return-Path. SPF failures often stem from misaligned or misconfigured bounce domains.

It's the "Return to Sender" address on the envelope. The post office uses it when delivery fails.