Senders: From, Sender

hechengjin

Senders: From, SenderFromThe 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.SenderThe value of a Sender field is tokenizable. It is a target. For example:     From: boss@heaven.af.mil     Sender: secretary@heaven.af.milAccording 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.


http://cr.yp.to/immhf/sender.html


Sender：邮件的发送者，就相当于是from的秘书，替from传达消息。
合法的信件头部必须包括 "From:" 行
      合法的信件头部也许会包括 "Sender:" 行
　  如果信件即包括 "From:" 又包括 "Sender:"，那么 "Sender:" 是发送者
　  如果信件包括 "From:" 但不包括 "Sender:"，那么 "From:" 里面的第一个地址是发送者
　  如果信件不包括 "From:"，那么这封信肯定不可能是 DomainKey 签名过的（包括有 "Sender:" 而无 "From:" 的情况）