What is the difference between envelope and header?

Email has two layers of addressing: the SMTP envelope and the message headers. They serve different purposes and can contain different information.

The envelope exists at the SMTP protocol level during transmission:

MAIL FROM: Where bounces go (return-path)

RCPT TO: Actual delivery destinations

The envelope controls routing. Servers use it to decide where to deliver the message and where to send bounce notifications.

The headers are part of the message content:

From: Who the recipient sees as the sender

To: Who the message appears addressed to

Cc, Bcc: Other visible/hidden recipients

Envelope and header addresses can differ legitimately. Mailing lists send from the list address (envelope) but show the original author (header). ESPs send from bounce-handling addresses (envelope) while displaying your brand (header).

Understanding this separation matters for authentication. SPF checks the envelope sender. DKIM signs headers. DMARC requires alignment between them.

The envelope is the shipping label; the letterhead is inside. Both matter, but for different reasons.