To header
The header field listing the message's primary recipients as the author addressed them; delivery itself follows the envelope, not this field.
What it means
RFC 5322 defines To as the primary recipients, Cc as others who receive a copy, and Bcc as recipients hidden from the rest. Delivery follows the envelope recipients given in RCPT TO, so the To field can name a mailing list, an empty group such as Undisclosed recipients:; or an address that never receives this copy.
Receivers still read it. Google's guidelines ask that To contain only actual recipients and appear only once. RFC 3834 recommends that out-of-office responders reply only when the recipient's address appears in To, Cc or Bcc, so mail that reaches people through the envelope alone draws fewer automatic replies.
Related terms
Sources
What this definition rests on. The wording is ours; the facts are theirs.
- RFC 5322, section 3.6.3: Destination Address Fieldsrfc-editor.org
- Google, Gmail Help: Email sender guidelinessupport.google.com
- RFC 3834, section 2: When (not) to send automatic responsesrfc-editor.org
All 432 terms in the glossaryThe wiki: blocklists and bounce codes