Sender header
The header field naming the mailbox that actually sent a message on the author's behalf, when that differs from the From address.
What it means
RFC 5322 separates authorship from transmission. From names the author; Sender names the agent responsible for sending, such as an assistant sending for a manager or a system sending for a user. Sender holds exactly one mailbox, must be present when From lists more than one, and should be left out when author and transmitter are the same. A submission server may add or replace it when it knows who is sending (RFC 6409).
DMARC looks at From, not Sender, so the Sender field carries no weight there. Mail programs may display it next to the author's name. Microsoft rejects messages that list several From addresses without a Sender with 550 5.1.20.
Related terms
On this site
Sources
What this definition rests on. The wording is ours; the facts are theirs.
- RFC 5322, section 3.6.2: Originator Fieldsrfc-editor.org
- RFC 6409, section 8.1: Add 'Sender'rfc-editor.org
- Microsoft Learn: Email non-delivery reports and SMTP errors in Exchange Onlinelearn.microsoft.com
All 432 terms in the glossaryThe wiki: blocklists and bounce codes