In SMTP, must the RCPT TO: and TO: match?

Question

When sending an email, the recipient list is given during the SMTP dialogue through RCTP TO: command. Later, in DATA command, header fields like 'To', 'Cc','bcc' are indicated. Does this RCPT TO list of recipients have to match with the headers indicated in DATA command?

Also, if the recipient is not indicated in RCPT TO, but in the To field of email header, is it going to be delivered to the recipient not in RCPT TO?

Answer 1

No, they don't have to match. When the message is sent, the SMTP Server (aka Message Transfer Agent or MTA) is creating a so called SMTP envelope which contains the recipients and the sender of the message (see 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.

It is, actually, quite often that the RCPT TO: Command has more recipients that the header of the message - one common case is the usage of "blind copies" bcc: (see 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".

Answer 2

Does this RCPT TO list of recipients have to match with the headers indicated in DATA command?

Nope.

if the recipient is not indicated in RCPT TO, but in the To field of email header, is it going to be delivered to the recipient not in RCPT TO ?

The RCPT. Here's a (modified) transcript from my own SMTP client where I do just what you ask:

CLIENT: MAIL FROM:<[email protected]> SERVER: 250 2.1.0 OK  CLIENT: RCPT TO:<[email protected]> SERVER: 250 2.1.5 OK  CLIENT: DATA SERVER: 354  Go ahead  CLIENT: Subject: Test email CLIENT: From:'John Doe'<[email protected]> CLIENT: To:'John Doe'<[email protected]> CLIENT: This is a test... CLIENT: . 

The message was successfully sent to "[email protected]".