Sendmail, 3rd Edition

Forwarding with Resent Headers

Some MUAs allow users to forward (resend, bounce, or redirect) messages to other users. For example, the mush(1) MUA forwards the current message to the user named fred with the following command:

message 1 of 3> m -f fred

Messages can also be forwarded with dist(1) from mh(1) and from within other MUAs.

When messages are forwarded, header lines that describe the forwarding user must begin with the Resent- prefix. When fred receives this message, he sees two similar header lines:

From: original-sender
Resent-From: forwarding-sender

When both the original From: and the forwarded Resent-From: appear in the same header, the Resent- form is always considered the most recent.

The sendmail program examines only a few header names to see whether a mail message has been forwarded. Those that it knows are listed in Table 25-11.

Table 25-11. Known resent headers

Resent- form of	Header
Resent-Bcc:	Bcc:
Resent-Cc:	Cc:
Resent-Date:	Date:
Resent-From:	From:
Resent-Message-ID:	Message-ID:
Resent-To:	To:

If sendmail finds any header with a name beginning with Resent-, it marks that message as one that is being forwarded, preserves all Resent- headers, and creates any needed ones.

Remove and Re-create the From: Header

Regardless of whether the message is forwarded, sendmail compares the sender envelope address to the address in the From: header (or Resent-From: if present). If they are the same, sendmail deletes the From: (or Resent-From:). The purpose ...