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Header fields

Name/value pairs. A core set provide email parameters. See RFC5322 2.2 https://tools.ietf.org/html/rfc5322#section-2.2

A list of registered headers and their usage is at RFC4021 https://tools.ietf.org/html/rfc4021

Custom headers can also be added using the extension-field prefix "x-" or "X-" See RFC822 4.7.4 https://tools.ietf.org/html/rfc822

Reply-To

Replies of email with Repy-To are addressed to address in Reply-To header.

Return-Path

The address of replies when there are message delivery issues. The final SMTP delivery agent inserts a header "Return-Path". This should not be generated by the original sender. New Return-Paths overwrite an existing one, so that there is never more than one. See RFC5321 4.4: https://tools.ietf.org/html/rfc5321#section-4.4

Sender

Sender usually refers to the email of the entity sending the email. This addressed is used in the 'From' header of outbound email.