sendmail, 4th Edition

Received:

Trace routing of mail RFC2822

The Received: header is used to record information about every site a mail message passes through on its way to ultimate delivery. First this header is inserted by the original sending site, then another is added by each site that the message passes through, including the site performing final delivery. Each new header is added to the list of Received: headers, forming a chronological record (reading bottom up through the headers) of how the mail message was handled.

The contents of the Received: header’s field are narrowly defined by RFC2821. The field’s defined form looks like this:

Received: "from" host "by" host ["via" atom] ["with" atoem]
       ["id" string] ["for" addr] ";" date
   ↑
   whitespace

The field is composed of six items that can be split over multiple lines by using whitespace to indent the second line. Each item is composed of two parts: a word (shown in quotation marks) and a value. Optional items are indicated by the enclosing square brackets in the previous example, but those brackets are not a part of the item and must be excluded when the item is actually used. Items, when present, must be in the following order:

from

Full canonical name of the sending host (required).

by

Full canonical name of the receiving host (required).

via

Physical network that was used to transmit the message, such as TCP, INTERNET, JANET, or XNS (optional).

with

Protocol used to receive the message, such as ESMTP or SMTP (optional).

id

Identifier assigned by the local ...