SMTP 允許在 RFC 中使用多個 FROM 地址。這曾經有用嗎，為什麼會存在？

根據 RFC，SMTP 允許在正文（而不是信封）上使用多個 FROM 地址。

此功能是否曾用於合法目的？

丟棄具有多個 FROM 地址的消息是否安全？

RFC 822實際上給出了這種用法的一個例子。它要求（第 4.4 節） Sender: 標頭在使用時存在。

     A.2.7.  Agent for member of a committee
     
             George's secretary sends out a message which was authored
        jointly by all the members of a committee.  Note that the name
        of the committee cannot be specified, since <group> names  are
        not permitted in the From field.
 
            From:   Jones@Host,
                    Smith@Other-Host,
                    Doe@Somewhere-Else
            Sender: Secy@SHost

已棄用它的RFC 2822繼續明確地允許這種特殊結構（第 3.6.2 節）。

from            =       "From:" mailbox-list CRLF

mailbox-list    =       (mailbox *("," mailbox)) / obs-mbox-list

在目前標準RFC 5322中，這沒有改變，並且仍然明確允許多個地址（第 3.6.2 節）。

   The from field consists of the field name "From" and a comma-
   separated list of one or more mailbox specifications.  If the from
   field contains more than one mailbox specification in the mailbox-
   list, then the sender field, containing the field name "Sender" and a
   single mailbox specification, MUST appear in the message.

它曾經有用嗎？是的，它仍然是，對於古代例子中顯示的那種場景。具有多個作者的郵件應該將所有作者都列在 From: 標題中，並將 Sender: 設置為在他們的電子郵件程序中實際點擊 Send 的人。

   The originator fields indicate the mailbox(es) of the source of the
   message.  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.  The "Sender:" field specifies the
   mailbox of the agent responsible for the actual transmission of the
   message.  For example, if a secretary were to send a message for
   another person, the mailbox of the secretary would appear in the
   "Sender:" field and the mailbox of the actual author would appear in
   the "From:" field.  If the originator of the message can be indicated
   by a single mailbox and the author and transmitter are identical, the
   "Sender:" field SHOULD NOT be used.  Otherwise, both fields SHOULD
   appear.

在公共 Internet 上的實踐中，執行此操作的消息並不常見，儘管它們確實發生在企業和學術環境中，其中一個人代表另一個人或一組人發送電子郵件更為常見。

我實際上從未見過這樣做的垃圾郵件（並且通過了我所有的其他控制項）。我通常認為丟棄或提高此類郵件的垃圾郵件分數是不安全的。

引用自：https://serverfault.com/questions/554520