Format of a mail message
At the top of every mail message is a highly structured header. Many programs expect the header to carry certain information, as described below. The main function of qmail-inject is to make sure that each outgoing message has an appropriate header.
For more detailed information, see http://pobox.com/~djb/proto/immhf.html.
A message contains a series of header fields, a blank line, and a body:
Received: (qmail-queue invoked by uid 666); 30 Jul 1996 11:54:54 -0000 From: [email protected] (D. J. Bernstein) To: [email protected] Date: 30 Jul 1996 11:54:54 -0000 Subject: Go, Bears! I've got money on this one. How about you? ---Dan (this is the third line of the body)
Each header field has a name, a colon, some contents, and a newline:
Subject: Go, Bears!
The field contents may be folded across several lines. Each line past the first must begin with a space or tab:
Received: (qmail-queue invoked by uid 666); 30 Jul 1996 11:54:54 -0000
The field name must not contain spaces, tabs, or colons. Also, an empty field name is illegal. qmail-inject does not allow field names with unprintable characters.
Case is irrelevant in field names: subject and SUBJECT and SuBjEcT have the same meaning.
Certain fields, such as To, contain address lists.
An address list contains some number of addresses or address groups, separated by commas:
[email protected], [email protected] (Somebody), A Person <[email protected]>, random group: [email protected], [email protected];, [email protected]
An address group has some text, a colon, a list of addresses, and a semicolon:
random group: [email protected], [email protected];
An address can appear in several forms. The most common form is [email protected].
Every address must include a host name. If qmail-inject sees a lone box name it adds the default host name.
All host names should be fully qualified. qmail-inject appends the default domain name to any name without dots:
It appends the plus domain name to any name that ends with a plus sign:
A host name may be a dotted-decimal address:
RFC 822 allows mailbox names inside angle brackets to include source routes, but qmail-inject strips all source routes out of addresses.
qmail-inject looks for sender address lists in the following fields: Sender, From, Reply-To, Return-Path, Return-Receipt-To, Errors-To, Resent-Sender, Resent-From, Resent-Reply-To.
If there is no From field, qmail-inject adds a new From field with the name of the user invoking qmail-inject.
RFC 822 requires that certain sender fields contain only a single address, but qmail-inject does not enforce this restriction.
qmail-inject looks for recipient address lists in the following fields: To, Cc, Bcc, Apparently-To, Resent-To, Resent-Cc, Resent-Bcc.
Every message must contain at least one To or Cc or Bcc. qmail-inject deletes any Bcc field. If there is no To or Cc field, qmail-inject adds a line
Cc: recipient list not shown: ;
This complies with RFC 822; it also works around some strange sendmail behavior, in case the message is passed through sendmail on another machine.
Every message must contain a Date field, with the date in a strict format defined by RFC 822. If necessary qmail-inject creates a new Date field with the current date (in GMT).
Every message should contain a Message-Id field. The field contents are a unique worldwide identifier for this message. If necessary qmail-inject creates a new Message-Id field.
Another important field is Received. Every time the message is sent from one system to another, a new Received field is added to the top of the message. qmail-inject does not create any Received fields.
A message is resent if it contains any of the following fields: Resent-Sender, Resent-From, Resent-Reply-To, Resent-To, Resent-Cc, Resent-Bcc, Resent-Date, Resent-Message-ID.
If a message is resent, qmail-inject changes its behavior as follows.
It deletes any Resent-Bcc field (as well as any Bcc field); if there are no Resent-To or Resent-Cc fields, qmail-inject adds an appropriate Resent-Cc line. It does not add a Cc line, even if neither To nor Cc is present.
If there is no Resent-From field, qmail-inject adds a new Resent-From field. It does not add a new From field.
qmail-inject adds Resent-Date if one is not already present; same for Resent-Message-Id. It does not add new Date or Message-Id fields.
Addresses are separated by commas, not spaces. When qmail-inject sees an illegal space, it inserts a comma:
djb fred -> djb, fred
qmail-inject removes all Return-Path header fields.
qmail-inject also removes any Content-Length fields.