email - 在 SMTP 中，RCPT 必须为 : and TO: match?

Question

发送电子邮件时，收件人列表在 SMTP 对话期间通过 RCTP TO: 命令给出。稍后，在 DATA 命令中，指示了诸如“To”、“Cc”、“bcc”之类的 header 字段。此 RCPT TO 收件人列表是否必须与 DATA 命令中指示的标题匹配？

此外，如果收件人未在 RCPT TO 中指定，但在电子邮件标题的 To 字段中，是否将发送给不在 RCPT TO 中的收件人？

Answer 1

不，他们不必匹配。当消息被发送时，SMTP 服务器(又名消息传输代理或 MTA)正在创建一个所谓的 SMTP 信封，其中包含消息的收件人和发件人(参见 RFC5321):

SMTP transports a mail object. A mail object contains an envelope and content. The SMTP envelope is sent as a series of SMTP protocol units (described in Section 3). It consists of an originator address (to which error reports should be directed), one or more recipient addresses, and optional protocol extension material.

实际上，RCPT TO: 命令的接收者通常多于消息的 header - 一种常见情况是使用“盲复制” bcc:(参见 RFC5321 ):

Addresses that do not appear in the message header section may appear in the RCPT commands to an SMTP server for a number of reasons. The two most common involve the use of a mailing address as a "list exploder" (a single address that resolves into multiple addresses) and the appearance of "blind copies".