So, over SMTP when a message is submitted, the SMTP envelope (sender, recipients, etc.) is different from the actual data of the message.
Sender header is used to identify in the message who submitted it. This is usually the same as the
From header, which is who the message is from. However, it can differ in some cases where a mail agent is sending messages on behalf of someone else.
Return-Path header is used to indicate to the recipient (or receiving MTA) where non-delivery receipts are to be sent.
For example, take a server that allows users to send mail from a web page. So,
firstname.lastname@example.org types in a message and submits it. The server then sends the message to its recipient with
From set to
email@example.com. The actual SMTP submission uses different credentials, something like
firstname.lastname@example.org. So, the
sender header is set to
email@example.com, to indicate the
From header doesn’t indicate who actually submitted the message.
In this case, if the message cannot be sent, it’s probably better for the agent to receive the non-delivery report, and so
Return-Path would also be set to
firstname.lastname@example.org so that any delivery reports go to it instead of the sender.
If you are doing just that, a form submission to send e-mail, then this is probably a direct parallel with how you’d set the headers.