D. J. Bernstein
Internet mail
Internet mail message header format

Senders: From, Sender

From

The value of a From field is tokenizable. It is a target list. Three examples:

     From: God@heaven.af.mil
     From: The Boss <God@heaven.af.mil>
     From: bill@irs.gov, audit@irs.gov

According to 822, the From field ``contains the identity of the person(s) who wished this message to be sent.'' According to 822bis, the From field ``specifies the author(s) of the message, that is, the mailbox(es) of the person(s) or system(s) responsible for the writing of the message.''

822 requires that every message contain

a From field whose value is a target, or
a From field and a Sender field.

This poses problems in practice for anonymous mail services. Readers can't assume that From conveys any useful information:

     From: Anonymous <nobody@nowhere.org>

People occasionally send messages without From fields. Outlook 98 (Corporate/Workgroup version, with Internet Mail Service, with the Outlook 98 Security Patch) omits From fields from local copies of outgoing messages.

Note that a From field cannot contain address groups:

     From: sender-not-shown: ;    (WRONG)

Pine 3.95 reportedly crashes when it tries to read this field.

Sender

The value of a Sender field is tokenizable. It is a target. For example:

     From: boss@heaven.af.mil
     Sender: secretary@heaven.af.mil

According to 822, the Sender field ``contains the authenticated identity of the AGENT (person, system or process) that sends the message.'' According to 822bis, the Sender field ``specifies the mailbox of the agent responsible for the actual transmission of the message.''

822 specified Sender, defaulting to From, as the target of delivery failure notices and other bounce messages. This use of Sender was already obsolete at the time: bounce messages are sent to the Return-Path address. It is unclear why Sender is supposed to be useful.

I recommend against all automated use of Sender. In practice some programs create Sender as

a copy of the envelope sender, or
a copy of From, or
the user's login name plus the current host name, without regard for whether this is a usable mail address.

None of these behaviors is useful. Local accounting information belongs in Received lines, not Sender lines.